Question

Name 2 functions of the thyroid hormone

Answer 1

Functions of thyroid hormones.

Thyroid hormones increase the basal rate of oxygen consumption and therefore, the basal metabolism of the tissues. This increased metabolism increases the rate of heat production. This is called calorigenic or thermogenic actions of thyroid hormones.

Increased metabolism increases body temperature. This causes vasodilation that decreases peripheral resistance and consequent changes occur in hemodynamics.

Thyroid hormones are essential for development of the central nervous system, especially during infancy and early childhood. Development of brain occurs maximally in last six months of fetal life and first six months of post natal life. During this period, thyroid hormones initiate and facilitate the process of differentiation and maturation of brain cells.

Thyroid hormones increase heart rate and myocardial contractility. Therefore, they increase the cardiac output.

Thyroid hormones stimulate lipolysis.

Thyroid hormones increase absorption of glucose from intestine.They increase hepatic glucose output by inducing gluconeogenesis. The also stimulate hepatic glycogenolysis. Therefore, plasma glucose level increases.

Thyroid hormones promote proteolysis in skeletal muscle, increasing the release of amino acid. They increase protein turnover by promoting protein degradation.

Thyroid hormones potentiate the metabolic actions of catecholamines, glucagons, cortisol and growth hormone, especially their gluconeogenic and lipolytic effects.

Thyroid hormones enhance the motility GI tract. There fore, hyperdefecation is a feature of hyperthyroidism and constipation is a feature of hypothyroidism. Thyroid hormone also increase appetite and food intake, and reab sorption of glucose from GI tract.

Thyroid hormones increase the expression of MHC genes in the skeletal muscle. Therefore, hypothyroidism is associated with muscle cramps and weakness. However, mus cle weakness also occurs in hyperthyroidism, which may partly be due to increased protein catabolism in the muscle. Myopathy occurs in chronic hyperthyroidism (thyrotoxic myopathy).

With respect to the reproductive system, in women, they cause follicular maturation and ovulation. Thyroid abnormalities in woman result in irregularities in menstrual cycle, such as menorrhagia (increased menstrual loss) in hypothyroidism and oligomenorrhea (decreased menstrual loss) in hyperthyroidism.

In males, they promote spermatogenesis. T3 promotes differentiation of prepubertal testicular Sertoli cells.

Thyroid hormones increase kidney size and promote growth of renal tubular epithelial cells. They increase renal blood flow and GFR. Tubular reabsorption of electrolytes, glucose and water is also increased by thyroid hormones. Increased water reabsorption increases blood volume.

In summary, thyroid hormones have widespread effects on metabolism, growth and development and control of many systemic functions.

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