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Understand Thyroid – A Functional Myriad, and Clear Skepticism on Hypothyroidism

Dr. K. V. Narasimha Raju & Dr. B. Swapna

Thyroid Disease is a hidden epidemic and arguably, among the commonest endocrine disorders worldwide. An estimated 108 million people in India suffer from endocrine and metabolic disorders among which 42 million people suffer from thyroid diseases. Early diagnosis and treatment remains the cornerstone of management.

Thyroid Disease is a hidden epidemic and arguably, among the commonest endocrine disorders worldwide. An estimated 108 million people in India suffer from endocrine and metabolic disorders among which 42 million people suffer from thyroid diseases. Thyroid diseases are different from other diseases in terms of their ease of diagnosis, accessibility of medical treatment, and the relative visibility that even a small swelling of the thyroid offers to the treating physician. Early diagnosis and treatment remains the cornerstone of management.

Physiology
The thyroid gland, located immediately below the larynx on each side of and anterior to the trachea, is one of the largest of the endocrine glands, normally weighing 15 to 20 grams in adults. The thyroid secretes two major hormones, thyroxine and triiodothyronine, commonly called T4 and T3, respectively. Both of these hormones profoundly increase the metabolic rate of the body. Complete lack of thyroid secretion usually causes the basal metabolic rate to fall 40 to 50 per cent below normal, and extreme excesses of thyroid secretion can increase the basal metabolic rate to 60 to 100 per cent above normal. The thyroid gland also secretes calcitonin, an important hormone for calcium metabolism.

Thyroid secretion is controlled primarily by thyroid-stimulating hormone (TSH) secreted by the anterior pituitary gland. Anterior pituitary secretion of TSH is controlled by a hypothalamic hormone, thyrotropin-releasing hormone (TRH). The rate of thyroid hormone production is controlled by the pituitary gland. If there is an insufficient amount of thyroid hormone circulating in the body to allow for normal functioning, the release of TSH is increased by the pituitary gland in an attempt to stimulate more thyroid hormone production. In contrast, when there is an excessive amount of circulating thyroid hormone, TSH levels fall as the pituitary attempts to decrease the production of thyroid hormone. In persons with hypothyroidism, there is a persistent low level of circulating thyroid hormones.

About 93 per cent of the metabolically active hormones secreted by the thyroid gland is thyroxine, and 7 per cent triiodothyronine. However, almost all the thyroxine is eventually converted to triiodothyronine in the tissues, so that both are functionally important. The functions of these two hormones are qualitatively the same, but they differ in rapidity and intensity of action. Triiodothyronine is about four times as potent as thyroxine, but it is present in the blood in much smaller quantities and persists for a much shorter time than does thyroxine.

The general effect of thyroid hormone is to activate nuclear transcription of large numbers of genes. Therefore, in virtually all cells of the body, great numbers of protein enzymes, structural proteins, transport proteins, and other substances are synthesized. The net result is generalized increase in functional activity throughout the body.

Iodine Requirement For Hormone Formation: To form normal quantities of thyroxine, about 50 milligrams of ingested iodine in the form of iodides are required each year, or about 1 mg/week. To prevent iodine deficiency, common table salt is iodized with about 1 part sodium iodide to every 100,000 parts sodium chloride. Iodides ingested orally are absorbed from the gastrointestinal tract into the blood in about the same manner as chlorides. Normally, most of the iodides are rapidly excreted by the kidneys, but only after about one fifth are selectively removed from the circulating blood by the cells of the thyroid gland and used for synthesis of the thyroid hormones.

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