Thyroid hormone is the hormone that controls your body's metabolism, the process in which your body transforms the food you eat into energy. The two main hormones your thyroid releases — thyroxine (T4) and triiodothyronine (T3) — collectively make up thyroid hormone.

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The thyroid hormone is well known for controlling metabolism, growth, and many other bodily functions. The thyroid gland, anterior pituitary gland, and hypothalamus comprise a self-regulatory circuit called the hypothalamic-pituitary-thyroid axis. The main hormones produced by the thyroid gland are thyroxine or tetraiodothyronine (T4) and triiodothyronine (T3). Thyrotropin-releasing hormone (TRH) from the hypothalamus, thyroid-stimulating hormone (TSH) from the anterior pituitary gland, and T4 work in synchronous harmony to maintain proper feedback mechanisms and homeostasis. Hypothyroidism, caused by an underactive thyroid gland, typically manifests as bradycardia, cold intolerance, constipation, fatigue, and weight gain. In contrast, hyperthyroidism caused by increased thyroid gland function manifests as weight loss, heat intolerance, diarrhea, fine tremor, and muscle weakness.

Iodine is an essential trace element absorbed in the small intestine. It is an integral part of T3 and T4. Sources of iodine include iodized table salt, seafood, seaweed, and vegetables. Decreased iodine intake can cause iodine deficiency and decreased thyroid hormone synthesis. Iodine deficiency can cause cretinism, goiter, myxedema coma, and hypothyroidism.[1][2][3]

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Function

The physiological effects of thyroid hormones are listed below:

• Increases the basal metabolic rate

• Depending on the metabolic status, it can induce lipolysis or lipid synthesis.

• Stimulate the metabolism of carbohydrates

• Anabolism of proteins. Thyroid hormones can also induce catabolism of proteins in high doses.

• Permissive effect on catecholamines

• In children, thyroid hormones act synergistically with growth hormone to stimulate bone growth.

• The impact of thyroid hormone on CNS is important. During the prenatal period, it is needed for the maturation of the brain. In adults, it can affect mood. Hyperthyroidism can lead to hyperexcitability and irritability. Hypothyroidism can cause impaired memory, slowed speech, and sleepiness.

• Thyroid hormone affects fertility, ovulation, and menstruation.