Thyroid hormones cardiovascular system

There is no discrete target tissue for thyroid hormones virtually every cell in the body is affected by thyroid hormones in some way. These hormones are intimately involved in the maintenance of normal function in virtually every cell type, including cellular responsiveness to other hormones, to the availability of metabolic substrates, to growth factors, and so on. Thyroid dysfunction can produce dramatic changes in the metabolism of proteins, carbohydrates, and lipids at the cellular level that can have repercussions for the operation of the cardiovascular, gastrointestinal, musculoskeletal, reproductive, and nervous systems. Some of the clinical manifestations of thyroid dysfunction are presented next in the discussions of hypothyroid and hyperthyroid states. 

Danzi S, Klein I. Thyroid hormone and the cardiovascular system. Minerva Endocrinol. 2004 29 139-150. 

Thyroid hormone has multiple effects on the cardiovascular system with various physiological consequences. Several genes that encode important regulatory and structural proteins in the heart have been shown to be thyroid hormone responsive. Thyroid hormone increases cardiac contractility, induces vasorelaxation and angiogenesis, prevents fibrosis and has favorable effects on lipid metabolism (reviewed by Pantos73). 

I. Klein and K. Ojamaa Thyroid Hormone and the Cardiovascular System. New England Journal of Medicine 344,501 (2001). 

The initial dose of levothyroxine is dependent on the patient s age, and the presence of associated disorders, as well as the severity and duration of hypothyroidism. In young patients with longstanding disease and patients over age 45 without known cardiac disease, therapy should be initiated with 50 meg daily of levothyroxine and increased to 100 meg daily after 1 month. The recommended initial daily dose for older patients or those with known cardiac disease is 25 meg per day titrated upward in increments of 25 meg at monthly intervals to prevent stress on the cardiovascular system. Some patients may experience an exacerbation of angina with higher doses of thyroid hormone. Although the TSH is very sensitive for under- or overreplacement, chnicians often fail to alter the dose of T4 based on TSH clearly outside of the normal range. ... 

In vivo effects of thyroid hormone and iodine intake on the cardiovascular system... 

I. Mechanism of toxicity. Excessive thyroid hormone potentiates adrenergic activity in the cardiovascular, gastrointestinal, and neurologic systems. The effects of T3 overdose are manifested within the first 6 hours after ingestion. In contrast, symptoms of T4 overdose may be delayed 2-5 days after ingestion while metabolic conversion to T3 occurs. 

The heart, as an endocrine organ itself, secretes the atrial natriuretic peptide, which has several important influences on the cardiovascular system. In addition to atrial natriuretic peptide, catecholamines, thyroid hormones, mineralocorticoids, sex steroids, and angiotensin II are other hormones also known to exert their effects on the cardiovascular functions and blood pressure. Hormones regulate the contraction and dilatation of the vascular bed. They may also affect the contraction of other smooth muscles other than the vascular smooth muscle. Oxytocin stimulates contraction of the myoepithelium in the mammary gland that is necessary for milk ejection. 

Uses Thyroid hormones (levothyroxine, T4, and liothyronine, T3) are used to treat overt hypothyroidism. In addition, supra-physiological doses of levothyroxine are widely used to reduce the risk of thyroid cancer recurrence by suppressing serum TSH concentrations. The use of levothyroxine suppressive therapy to induce shrinkage of benign thjnoid nodules remains controversial and its routine use is not recommended in view of its low efficacy and potential long-term effects on the cardiovascular system and the skeleton caused by the induction of subchnical hjrper-thyroidism [5 ]. 

---