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Published in final edited form as:
Am J Med. 2014 August ; 127(8): 691–698. doi:10.1016/j.amjmed.2014.03.009.
Thyroid and the Heart
Ira Martin Grais1 and James R. Sowers2,3,4,5
1Department
of Medicine/Cardiology Division, Northwestern Feinberg School Medicine, Chicago,
IL
2Department
of Internal Medicine, University of Missouri School of Medicine, Columbia, MO
3Department
of Medical Pharmacology and Physiology, University of Missouri School of
Medicine, Columbia, MO
4Department
of Diabetes and Cardiovascular Center, University of Missouri School of Medicine,
Columbia, MO
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5Harry
S Truman Veterans Affair Medical Center, Columbia, MO
Abstract
Thyroid hormones modulate every component of the cardiovascular system necessary for normal
cardiovascular development and function. When cardiovascular disease is present, thyroid
function tests are characteristically indicated to determine if overt thyroid disorders or even
subclinical dysfunction exists. As hypothyroidism, hypertension and cardiovascular disease all
increase with advancing age monitoring of TSH, the most sensitive test for hypothyroidism, is
important in this expanding segment of our population. A better understanding of the impact of
thyroid hormonal status on cardiovascular physiology will enable health care providers to make
decisions regarding thyroid hormone evaluation and therapy in concert with evaluating and
treating hypertension and cardiovascular disease. The goal of this review is to access
contemporary understanding of the effects of thyroid hormones on normal cardiovascular function
and the potential role of overt and subclinical hypothyroidism and hyperthyroidism in a variety of
cardiovascular diseases.
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Keywords
Thyroid dysfunction; cardiac output; heart failure; peripheral vascular function; atrial fibrillation;
coronary artery disease
© 2014 Elsevier Inc. All rights reserved.
CORRESPONDING AUTHOR: Ira Martin Grais, MD, 6611 N. Central Park Avenue, Lincolnwood, IL 60712-3701, igrais@sbcglobal.net.
Publisher's Disclaimer: This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our
customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of
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discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Both authors had access to the data in this manuscript and both were the sole authors.
Grais and Sowers
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INTRODUCTION
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The relationship of thyroid hormonal abnormalities and cardiovascular disease goes well
beyond the risk of atherosclerosis in association with hypothyroidism and the risk of atrial
fibrillation in persons with hyperthyroidism.1 The two organ systems are intimately linked
by their embryological anlage, and the ubiquitous effects of thyroid hormone on the major
components of the entire circulatory system: the heart, the blood vessels and the blood as
defined by the flow law (Fig 1).2 Cardiac output is normally modulated by peripheral
arteriolar vasoconstriction and dilatation, venous capacitance, and blood volume in response
to tissue metabolic needs.3 The heart can only pump the blood that returns to it, so factors
that influence venous return such as blood volume and venous capacitance are critical.
Arteriolar dilatation reduces peripheral vascular resistance and thus afterload, increasing
cardiac output. The four key issues to be emphasized in this review include a discussion of
the normal effects of thyroid hormone on cardiovascular function, as well as therapeutic
strategies designed to manage coronary artery disease, atrial fibrillation and heart failure
when thyroid hormonal dysfunction is present. Before discussing these clinical issues, a
brief summary of the thyroid hormone metabolic effects on the heart and vasculature will be
reviewed.
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CARDIOVASCULAR PHYSIOLOGY
In reviewing the thyroid and the circulatory system, certain key concepts are worth restating
and relating to the flow law as illustrated in Figure 1. As described4, thyroid hormone causes
a myriad of hemodynamic effects and all can be related directly or indirectly to the flow law.
Thyroid function influences every structure of the heart and its specialized conducting
system. Moreover, thyroid hormones, in addition to their direct effects on cardiovascular
function also have indirect effects mediated through the autonomic nervous system, the
renin-angiotensin-aldosterone system (RAAS), vascular compliance, vasoreactivity and
renal function.
THYROID HORMONE EFFECTS ON THE CARDIOVASCULAR SYSTEM
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The major effects of thyroid hormones on the heart are mediated by triiodothyronine (T3)
(Fig 2). Indeed, T3 generally increases the force and speed of systolic contraction and the
speed of diastolic relaxation.5 In addition, T3 decreases vascular resistance, including
coronary vascular tone, and increases coronary arteriolar angiogenesis.5 These multiple
thyroid hormone effects are largely mediated by the action of nuclear based thyroid hormone
receptors (TR), specifically the TR and . TR is the predominant TR isoform in the heart,
and it is the predominant subtype through which T3 binds to nuclear TRs and signals in
cardiomyocytes.5–8 T3-activated TR cardiomyocyte growth and maturation is mediated by
phosphorylation/activation of phosphoinositol 3-kinase (PI3-K), protein kinase B (Akt), and
mammalian target of rapamycin (mTOR) which promotes a number of developmental
processes including titan (sarcomere protein) transition.9–13 These T3-activated TR growth
effects are modulated by increases in atrial natriuretic peptide (ANP) and decreases in
protein kinase C (PKC), especially PKCe.11,12 T3 mediated activation of these signaling
pathways initiates changes in gene expression which are compatible with the physiological
cardiac hypertrophy phenotype. T3-activated TR regulates myosin heavy chain (MHC)
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genes which encode for the two contractile protein isoforms of the thick filament of the
cardiomyocyte.5–10 T3 exerts a positive effect on the transcription of the myosin heavy
chain(MHC) gene and a negative effect on the MHC gene expression (Fig 2).5–10 MHC
expression is modulated by T3 regulation of micro (m)-RNAs which influence MHC mRNA
turnover and translation.
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Thyroid hormones can promote both physiological and pathological myocardial
hypertrophy. In this regard cardiac hypertrophy, in its initial phases, presents a physiological
process with includes increased adenosine triphosphatase (ATP) and gene expression of the
sarcoplasmic reticulum Ca2+ (SERCa2+) and decreased expression of MHC (Fig 2). T3activated TR cardiac effects also include the regulation of cation transport (Fig 2).
Regulation of intracellular Ca2+ ([Ca2+]i) is important for both normal systolic and diastolic
function. For example, T3 promotes increases in SERCa2+ ATPase and the ryanodine
channel, and decreases phosphorylation/activation of phospholamban which functions to
inhibit the SERCa2+ pump.14–18 Diastolic function of the heart is substantially influenced by
the thyroid status. The speed of diastolic relaxation in the heart is markedly influenced by
lowering of the [Ca2+]i levels. In cardiomyocytes, most [Ca2+]i lowering is achieved by
pumping [Ca2+]i into the sarcoplasmic reticulum by the SERCa2+ pump. Experimental
results in animal models of hypothyroidism indicate that the level and activity of the
SERCa2+ pump is markedly decreased and that of inhibitory phospholamban increased.5
These SERCa2+ and phospholamban changes can be linked to a decrease in the rate of
diastolic relaxation. The ryanodine receptor is also decreased in hypothyroid hearts.5
Finally, the 1 adrenergic and the TR receptors are positively and negatively regulated by
T3, respectively, which promotes optimal modulation of T3-activated TR inotrophic and
chronotrophic cardiac effects.5
MECHANISMS OF THYROID HORMONE EFFECTS ON THE VASCULATURE
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Thyroid hormones exert effects on the vasculature that generally lead to reduced vascular
tone and maintenance of normal arteriolar remodeling.5 It has been known for two decades
that T3 exerts direct effects on vascular smooth muscle cells to promote relaxation.5 Several
mechanisms for this T3 mediated vascular relaxation have been reported. For example, it has
been demonstrated that T3 dose-dependently reduces expression of the angiotensin (Ang) II
type 1 receptor and reduces the increased [Ca2+]i and contractile response to Ang II.19.
Further, T3 stimulates nitric oxide (NO) production via activation of the PI3-K/Akt
mediated endothelial NO synthase (eNOS) signaling pathway.20, 21 The resulting increase in
bioavailable NO is associated with decreased myosin light chain (MLC) phosphorylation in
response to Ang II and phenylephrine.21 Collectively, these data suggest that T3 reduces
VSMC contraction by decreasing [Ca2+]i as well as Ca2+ sensitization. Studies have shown
that T3 also promotes angiogenesis and increases the density of small arterioles, including
coronary arterioles.5, 22, 23 This T3-activated TR effect on coronary arterioles may be
especially important following myocardial ischemia and in the process of myocardial
ischemic reconditioning.
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THYROID HORMONES AND HEART FAILURE
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The role of low thyroid hormone function in promoting heart failure and the potential
benefits of thyroid hormone replacement, have been reviewed extensively.5, 24 In this
regard, heart failure can lead to the down-regulation of the thyroid hormone signaling
system in the heart.5 In the failing heart, decreases of nuclear TR levels occur. In addition,
serum levels of T4 and T3 are decreased with heart failure in the context of the nonthyroidal illness syndrome. In animal models, it can be shown that in pressure overloadinduced cardiac hypertrophy, a decrease of TR levels occurs. Heart failure is an increasing
medical problem in our aging population. There is increasing evidence that decreased
thyroid function may contribute to systolic and diastolic dysfunction.5,22,25 Data from
clinical studies indicate that thyroid hormone replacement in patients with heart failure has
beneficial effects on cardiac contractile function.25–28 Overall, it appears that in heart
failure, a hypothyroid cardiac state may occur due to decreased TR levels in failing
hearts.5, 25–28 Animal studies and a limited number of human trials indicate that increasing
thyroid hormone action, either by increasing T3 receptor levels or serum levels of T3
hormone itself, can improve cardiac function without significant detrimental effects.24, 29,30
It is currently unclear if long term administration of thyroid hormone to patients in heart
failure will be well tolerated and will lead to increased survival. This can only be determined
by long term randomized controlled clinical trials.
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While atherosclerosis and atrial fibrillation are most commonly related to abnormal thyroid
function, numerous other cardiac conditions have also been related to thyroid dysfunction.
These include pericarditis, pericardial effusion,31 cardiac tamponade, sinus bradycardia and
tachycardia, atrioventricular block,32–34 Torsade de pontes ventricular tachycardia typically
with a long QTc, left ventricular systolic and diastolic dysfunction, heart failure, high output
congestive state, cardiomyopathy,35 mitral valve prolapse (in particular with autoimmune
thyroid gland disorders),36,37 endothelial dysfunction, dyslipidemia, and both systolic and
diastolic hypertension.38 Thyroid hormones exert effects on both the heart and the vascular
system as discussed above. Hypothyroidism decreases endothelial mediated vasorelaxation
and vascular compliance and thus elevated diastolic blood pressure (BP).39 Lowered
peripheral vascular resistance in hyperthyroidism increases blood volume and venous
return.40 This can lead to what is called “ high output failure” when a more accurate term is a
congestive state. Clinically relevant heart failure implies that despite adequate venous return
the heart cannot pump all the blood that returns to it. However, this is not the case in
uncomplicated hyperthyroidism where there is a high output state not unlike that which may
occur with a peripheral arteriovenous fistula, severe anemia, pregnancy, or severe liver
disease. While these cardiovascular disease abnormalities have been described with overt
thyroid dysfunction, some are increasingly recognized as being associated with subclinical
hypothyroidism and subclinical hyperthyroidism. Even high normal thyroid hormone
function is associated with a slightly increased risk for developing atrial fibrillation.35
HYPOTHYROIDISM
Hypothyroidism is characterized by depressed levels of T4 and T3 with compensatory high
levels of thyroid stimulating hormone. In seeking the classic clinical manifestations of this
condition such as fatigue, sluggishness, hoarse voice, constipation, delayed distal tendon
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reflexes, and skin changes, the clinician should also evaluate patients for cardiovascular
manifestations of hypothyroidism. The most common are diastolic hypertension, sinus
bradycardia, due to sinus node dysfunction, and failure of the sinus node to accelerate
normally under conditions of stress such as caused by fever, infection or heart failure.41
Other cardiac manifestations may include heart block, pericarditis, pericardial effusion, rare
cardiac tamponade.42 Additionally, in chronic hypothyroid states there is increased risk of
atherosclerosis often associated with dyslipidemia (hypercholesterolemia) and hypertension.
Less common are cardiomyopathy, endocardial fibrosis, and myxomatous valvular changes.
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The coronary artery disease accompanying hypothyroidism may be pre-existent or be
aggravated by the thyroid dysfunction especially as peripheral vascular resistance increases.
The hypertension associated with hypothyroidism may be asymptomatic or attended by
overt myocardial ischemia including angina pectoris or myocardial infarction. Great caution
is needed in treating such patients with thyroid hormone replacement. The key with
replacement therapy is to “ go low and go slow.” The apt caveat is to start with the lowest
imaginable dose of thyroid hormone and cut that in half. Important exceptions are patients
who are young and without coronary risk factors or patients immediately after total
thyroidectomy. There are, of course, many causes of hypothyroidism and these will
influence the judgments made about therapy. These are addressed elsewhere in this
supplement; however, it is important to identify autoimmune thyroid disorders such as
Hashimoto’ s thyroiditis and Graves’ disease since these require special therapeutic
considerations.
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Typical electrocardiographic changes that can be seen in hypothyroidism include sinus
bradycardia, a prolonged QTc (which can result in Torsade de Ponte ventricular
tachycardia), low voltage, and the rare instance of atrioventricular block. Some of the salient
cardiovascular changes that can occur when hypothyroidism is present are sinus
bradycardia, decreased cardiac output, diastolic hypertension, increased myocardial oxygen
demand due to increased afterload, long QTc with increased risk of Torsade, increased risk
of atherosclerosis due to dyslipidemia (increased total cholesterol, increased low density
lipoprotein cholesterol, decreased low density lipoprotein receptors, hypertension and
elevated homocysteine levels), some evidence for increased abdominal aortic atherosclerosis
and increased intimal-medial carotid thickening, and decreased myocardial perfusion which
can resolve with thyroid replacement therapy. Some of these changes are risk factors for
coronary artery disease, some relate to the flow law, and some are prime determinants of left
ventricular function and myocardial oxygen demand.
When coronary artery disease is known or suspected to be present, treating hypothyroidism
is a challenge for the clinician. Key questions include 1) What dose of thyroid replacement
is best if coronary artery disease is known, 2) What dose if coronary artery disease is
suspected, and 3) Does the patient need risk stratification for revascularization before
thyroid replacement therapy is initiated? Some of the predominant pathophysiologic and
therapeutic considerations with thyroid hormone replacement include the fact that there is
increased maximum oxygen consumption (M O2) in the setting of increased peripheral
vascular resistance. Secondly, in hypothyroid patients with unstable angina, main left
anterior descending coronary disease, triple vessel disease with impaired left ventricle
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function and with overt hypothyroidism, angioplasty or coronary artery bypass grafting,
merit consideration before thyroid hormone replacement therapy. For hypothyroid patients
with stable CAD, one should use lower doses of L-thyroxin and increase the dose slowly.
For example one may consider starting at 12.5 μg orally daily and increasing the dose every
six weeks. The lowering of peripheral vascular resistance with thyroid hormone replacement
can also ameliorate the myocardial ischemia in patients with hypothyroidism. The goal of
therapy is a euthyroid state with normal TSH and, of course, improvement in myocardial
ischemia and cardiac function.
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Approximately 4% of patients with hypothyroidism develop pericarditis so that the finding
of pericarditis warrants checking a TSH level even if there is another obvious cause of any
non-traumatic pericarditis. Patients with pericarditis require observation for effusion or
tamponade although these can occur without pericarditis. The cause of pericardial effusion
in hypothyroidism is not certain but likely due to volume retention. The fluid is extruded
from the surface of the heart43 and may be golden in color due to cholesterol crystals.44
Subclinical hypothyroidism is defined as a state with high TSH with normal blood levels of
T4 and T3. Here the thyroid dysfunction is compensated by the greater stimulation of the
elevated TSH level. Despite normal levels of thyroid hormone, such patients are at
somewhat increased risk of atherosclerosis. The clinical decision about starting thyroid
supplement therapy will be influenced by the age of the patient, the cause of the
hypothyroidism and the presence of other atherosclerotic risk factors including hypertension
and dyslipidemia. These risk factors for atherosclerosis require appropriate medical
management along with thyroid supplementation. With the aging of our population and the
ease of getting a TSH level, we can expect to see an increased incidence of subclinical
hypothyroidism.45 Endothelial dysfunction is a known early progenitor of hypertension and
atherosclerosis. There is evidence of decreased nitric oxide (NO) mediated vascular
relaxation in patients with subclinical hypothyroidism as demonstrated by abnormal flow
mediated vasodilatation.46 Flow mediated vasodilatation depends on the presence of
adequate bioavailable NO in the endothelium. Evaluation of endothelial mediated vascular
relaxation has revealed reduced flow mediated vasodilatation in persons with subclinical
hypothyroidism. Baseline and flow mediated (NO dependent) vasodilatation values were
significantly higher in persons with subclinical hypothyroidism after treatment with Lthyroxin. This study and another in a young cohort47 support the notion that thyroid
replacement therapy is beneficial in patients with subclinical hypothyroidism. Left and right
ventricular systolic and diastolic dysfunction have also been described in subclinical
hypothyroidism and there is evidence for improvement in ventricular function with thyroid
replacement therapy.48–50
HYPERTHYROIDISM
Hyperthyroidism is characterized biochemically by a low TSH level and elevated T4, T3 or
both. The causes include Graves’ disease, nodular thyroid disease, and factitious or
iatrogenic over dosage with thyroid hormone.51 Patients with hyperthyroidism can develop a
life threatening complication called thyroid storm or crisis, requiring urgent therapy with
beta blockers, antithyroid medication and iodine. This complication can be precipitated by
an acute illness such as a myocardial infarction, infection or other stress.52 Timely treatment
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of this condition is especially critical in patients with underlying coronary disease or heart
failure.
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The clinical symptoms and signs of hyperthyroidism include systolic hypertension,
increased left ventricular mass,53 exercise intolerance, angina pectoris and systolic
murmurs.53, Complications include atrial fibrillation with its risk of stroke, and high output
and heart failure.54, 55 A serum TSH level should be measured in any patient with
paroxysmal or sustained atrial fibrillation. If the TSH level is low, further thyroid evaluation
is needed. Atrial fibrillation, especially in the presence of pre-existent heart disease can
result in clinical heart failure. This heart failure may be due to an associated rapid
ventricular response which when sustained can lead to tachycardia-mediated
cardiomyopathy. The loss of atrial contractile function and decreased diastolic filling time
due to the tachycardia may cause increased filling pressures further contributing to this
cardiomyopathy.
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Atrial fibrillation and atrial flutter management presents unique challenges in patients with
associated hyperthyroidism. The usual guidelines should be followed except that efforts to
restore sinus rhythm are ordinarily delayed until the patient is euthyroid. This reduces the
likelihood of the rhythm reverting to atrial fibrillation. In the absence of evidence based
studies to support anticoagulation in such patients, careful clinical judgment is required.56
With novel anticoagulants such as the direct thrombin antagonist dabigatran and the factor
Xa inhibitors rivaroxaban and apixaban for nonvalvular atrial fibrillation, such a decision for
anticoagulant therapy is likely to be easier in the future, especially in patients who are not
good candidates for warfarin.
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While sinus tachycardia is the most common arrhythmia seen in hyperthyroidism, the
incidence of atrial fibrillation ranges from 2–20%, with prevalence increasing with age. Of
all cases of atrial fibrillation, only 1% are due to overt hyperthyroidism. Yet available
therapy justifies checking the TSH level in any patient with atrial fibrillation. The principle
objectives in treating atrial fibrillation associated with hyperthyroidism are rate control,
prevention of thromboembolism, and restoration of sinus rhythm. The following list is the
2011 guidelines of the joint committee of the American College of Cardiology and the
American Heart Association57: 1. Beta blockers to control the heart rate unless specifically
contraindicated; 2. When a beta blocker cannot be used, a nondihydropyridine calcium
channel antagonist is recommended; 3. oral anticoagulation (INR 2.0 to 3.0); 4. Once
euthyroid state is achieved, antithrombotic prophylaxis is the same as for patients without
hyperthyroidism.
In patients with atrial fibrillation/flutter, if digitalis is used for rate control, higher doses are
typically needed. Two thirds of patients return to sinus rhythm with radioiodine or
antithyroid drugs within 2–3 months. Despite its significant iodine content, amiodarone can
be used safely in these patients for rate control and cardioversion while checking the TSH
level at least every six months. The role of new anticoagulants such as dabigatran,
rivaroxaban and apixaban remain to be determined in the patient with both atrial fibrillation
and hyperthyroidism.
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Using the CHADS2 risk score provides useful guidelines for determining prophylactic
anticoagulation and compelling considerations in the management of atrial fibrillation with
regard to stroke prevention. For patients with borderline risk based on the CHADS2 score,
the newer CHA2DS2-VASc score assigns points for female gender, age 65–75, and vascular
disease. This latter score seems to fill an important gap.
The most evidence-based study available reached variant conclusions from that discussed
above59 regarding stroke in thyrotoxic atrial fibrillation.60 This evidence based study
involved a retrospective of 610 patients with untreated thyrotoxicosis, 91 or 14.9% of whom
had atrial fibrillation, the highest frequency being in elderly patients.60 The risk of
cerebrovascular events was calculated using logistic regression methods. Only age was an
important independent risk factor. The authors concluded that the indication for prophylactic
treatment with anticoagulants for prevention of stroke in thyrotoxic atrial fibrillation seemed
doubtful in the absence of controlled trials. Aspirin seems a good alternative in younger
patients without organic heart disease. Meanwhile the new novel anticoagulants may help
resolve the issues.61
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Heart failure in hyperthyroidism is another complex problem with many facets. It is
essential to distinguish a congestive state from clinical heart failure. If heart failure exists, a
number of conditions can precipitate it. Adding a Doppler echocardiogram to a careful
history and physical will usually clarify if structural heart disease or dysfunction is present.
The differential diagnosis of heart failure, with which the Doppler echocardiogram can
assist, includes high output failure (congestive state), tachycardia induced cardiomyopathy,
precipitation by atrial fibrillation, precipitation by coexistent organic heart disease including
coronary heart disease, hypertension, valvular disease including mitral valve prolapse, left
ventricular dilatation leading to mitral regurgitation and ruling out cardiac tamponade.
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The presence of hyperthyroidism does not exclude other common causes of heart failure.
Moreover, there is an essential basic principle that applies: whenever an organ or organ
system fails, look for a precipitating cause. Common causes of heart failure include onset of
atrial fibrillation, uncontrolled ventricular response with atrial fibrillation, uncontrolled
hypertension, excessive salt intake, failure to adhere to medical therapy, myocardial
ischemia or infarction, papillary muscle dysfunction, ruptured chordae, endocarditis, other
infections, renal failure, and, of course, hyperthyroidism. In 2009 the FDA approved the
Thyretain™ TSI Reporter Bioassay, the first test specifically detecting thyroid stimulating
immunoglobulin (TSI), the causative agent for Graves’ disease. Initial testing includes a
TSH, T3 and FT4 in asymptomatic or symptomatic patients at clinically increased risk of
hyperthyroidism. If the TSH is low or even borderline low, step two is to repeat the TSH to
confirm accuracy, and obtain TSI and thyroid peroxidase (TPO) antibody tests. If the TSI
test is negative, but TSH, T3 and/or FT4 are abnormal, the clinician should pursue further
cardiovascular workup and consider consultation with an endocrinologist.
When hyperthyroidism is suspected in a patient with atrial fibrillation, the TSI test should be
obtained and attempts to restore sinus rhythm should be deferred until the patient is rendered
euthyroid in order to reduce the risk of failure to convert or of recurrent atrial fibrillation.
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Beta blockers have a pre-eminent role in the management of heart failure in hyperthyroidism
although the ultimate treatment is restoration of a euthyroid state.
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Subclinical hyperthyroidism is defined as normal T4 and T3 levels with a low TSH level. As
with subclinical hypothyroidism, the ease of getting TSH levels results in an increased
recognition of subclinical hyperthyroidism, especially in the elderly. Such patients also have
an increased risk of developing atrial fibrillation. Indeed, the Rotterdam study showed that
even individuals with high normal thyroid function (high range of normal TSH levels) have
an increased such risk as well as for atherosclerosis and myocardial infarction in elderly
women.64,65,66 Such patients require periodic monitoring to search for evidence of overt
hyperthyroidism. A decision to treat subclinical hyperthyroidism depends on its cause,
evidence of cardiac disease, and comorbidities.
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While controversy remains for the management of both subclinical hyperthyroidism and
hypothyroidism, it was concluded in a recent review51 that a clinical decision regarding
initiating therapy requires consideration of the cause of the thyroid disease, the degree of
thyroid function tests abnormality, associated comorbidities, risk of progression, age of the
patient, and coexistent conditions such as pregnancy. The authors recommend the urgent
need for large-scale randomized trials. Meanwhile investigators,67 using data from 10
prospective cohort studies totaling 52,674 patients assessed the risks of coronary heart
disease mortality and events and atrial fibrillation in patients with subclinical
hyperthyroidism and concluded that risks were increased when the TSH level was lower
than 0.10 mU/L. In this regard, most patients with both primary hypothyroidism and Grave’ s
disease will have positive antibodies as iterated in two review articles.68, 69
SUMMARY
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Thyroid hormone affects virtually every anatomic and physiologic component of the
cardiovascular system. In the presence of heart disease, pericardial disease, heart failure
and/or arrhythmias, overt or subclinical thyroid dysfunction merits a high level of clinical
suspicion. The ease of obtaining a screening TSH level especially in our aging population
means many more patients will need assessment, risk stratification, and treatment in the
future. By understanding pertinent cardiovascular physiology and pathophysiology,
physicians will have a firmer basis for making the often complicated recommendations for
patient care even when evidence-based studies are not yet available. Meanwhile for
subclinical thyroid disease, while routine treatment remains controversial, routine screening
with TSH levels merits implementation especially in pregnant women, women over 60 years
of age, and anyone whose risk of thyroid dysfunction is high.70
Acknowledgments
IRA MARTIN GRAIS:
Funding related to this manuscript: None
Conflict of interest: None
JAMES R. SOWERS:
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Funding related to this manuscript: Work conducted in our laboratory is supported by NIH (R01 HL73101-08 and
R01 HL107910-03) (JRS) and Veterans Affairs Merit System 0018 (JRS).
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Conflict of interest: None
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CLINCIAL SIGNIFICANCE
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•
Thyroid and cardiovascular function are intimately linked.
•
When thyroid dysfunction is known or suspected, cardiovascular disease or risk
should be assessed.
•
When certain cardiovascular diseases, such as atrial fibrillation or sinus
bradycardia occur, thyroid function should be assessed.
•
Cardiac and peripheral vascular function, including cardiac and endothelial
mediated vasorelaxation, is partly dependent on thyroid hormone signaling.
•
Subclinical thyroid dysfunction can also be associated with cardiac disorders
and merits clinical screening.
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Figure 1. Each component of the flow law is influenced by thyroid hormone
MAP = mean arterial pressure, RAP = right atrial pressure, = Cardiac Output, TPR = total
peripheral resistance. The Poiseuille-Hagan Law demonstrates how small changes in
arteriolar radius lead to geometric changes in arteriolar resistance. R= resistance; r = radius;
# = (η = dynamic fluid viscosity; Δ = change; X = distance in direction of flow; ∏ =
mathematical constant Pi).
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Figure 2. Thyroid hormone effects on the heart
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T3 = triiodothyronine; TR = thyroid hormone receptors; cAMP = cyclic AMP, PKC =
protein kinase C; PI3-K = phosphoinositol 3-kinase; Akt = protein kinase B; (p) mTOR =
phosphorylation of mammalian target of rapamycin; (p)ERK = phosphorylation of
extracellular-signal-regulated kinases; SERCa2+= sarcoplasmic reticulum Ca2+; Na =
sodium; K-ATPase = potassium adenosine triphosphatase; MHC = myosin heavy chain
alpha; MHC = myosin heavy chain alpha beta; ANP = atrial natriuretic peptide
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