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Thyroid Gland
The thyroid gland develops in the fetal life from a median outgrowth of the floor of pharynx (windpipe), descends into the front of the neck before birth where it bifurcates and forms the thyroid lobes. Normally the thyroid gland is situated in the front of the neck but sometimes it is partially or wholly behind the sternum (breast bone). The normal adult thyroid gland consists of two lateral lobes joined at the lower poles by an isthmus (thyroid tissue) and weighs between 20 to 30 grams.
The function of the thyroid gland, which is the largest endocrine gland in the body, is to synthesize, store and secrete the three thyroid hormones:
Thyroid Gland
- Thyroxine (T4) and
- Tri-iodothyronine (T3)
The thyroid gland releases T3 and T4 into the blood stream which transports them through the whole body. These hormones, then, control the metabolism (conversion of calcium and oxygen to energy). The normal thyroid gland produces a mixture of hormones which contain about 80% T4 and only 20% T3. However, T3 is about 4 times stronger than T4. The thyroid controls the rate at which the body burns off energy, synthesizes essential proteins, and the body's sensitivity to other hormones. The thyroid gland also produces the hormone calcitonin, which plays a role in calcium homeostasis.
Iodine is an essential constituent for the secretion of these hormones in the thyroid gland. The amount of iodine ingested daily is normally 100-200 micrograms, and this is derived from marine fish, vegetables grown in iodine-containing soil, meat, cow's milk, water and table salt to which iodine or iodate has been added. The formation of thyroid hormones requires adequate amounts of dietary iodine. Iodine has been absorbed from the gastrointestinal tract as iodide and is concentrated from the blood serum by the thyroid gland. This trapping of iodide is enhanced by the thyroid stimulating hormones and inhibited by thiocyanates and perchlorates. When daily intake of iodine falls below about 100 micrograms, the hormonal output is reduced.
The level of serum cholesterol is, in part, controlled by the thyroid hormones. The physiological effects of the hormones secreted by the thyroid gland are skeleton growth, sexual maturation and mental development.