Review
Thyroid and the Heart

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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 thyroid-stimulating hormone, 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 about 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.

Section snippets

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 described,4 thyroid hormone causes myriad 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

Thyroid Hormone Effects on the Cardiovascular System

The major effects of thyroid hormones on the heart are mediated by triiodothyronine (T3) (Figure 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 TRα and

Mechanisms of Thyroid Hormone Effects on the Vasculature

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 2 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+

Thyroid Hormones and Heart Failure

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 downregulation 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

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 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

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,

Conclusion

Thyroid hormone affects virtually every anatomic and physiologic component of the cardiovascular system. In the presence of heart disease, pericardial disease, heart failure, 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

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    Funding: Work conducted in our laboratory is supported by the National Institutes of Health (R01 HL73101-08 and R01 HL107910-03) (JRS) and Veterans Affairs Merit System 0018 (JRS).

    Conflict of Interest: None.

    Authorship: Both authors had access to the data in this manuscript and both were the sole authors.

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