The thyroid is a gland located on the anterior (front) portion of the neck attached to the upper part of the trachea (windpipe). The thyroid is a bi-lobed gland. This gland is small in size, about 4 centimeters long and 1-2 centimeters wide.  

The thyroid produces and secretes biologically important hormones. Tissue in the thyroid is made up of two different kinds of cells: follicular and parafollicular cells. The thyroid is composed mostly of follicular cells which secrete T3 and T4 hormones. The T4 hormone (thyroxine) and T3 hormone (triiodothyronine) is derived from the amino acid tyrosine during iodination of the amino acid. Parafollicular cells secrete the hormone calcitonin. Iodine is important in the function of the thyroid gland. Iodine is a chief component of the hormones produced by the thyroid gland. Iodine deficiency can cause thyroid dysfunction, hence the need for iodized salt. The thyroid also produces and secretes the hormone calcitonin. The hormone calcitonin decreases plasma calcium ions concentration by inhibiting the release of calcium ions from the bone. Calcitonin secretion is regulated by plasma calcium ion levels.

The thyroid plays a key role in regulating the body’s metabolism. What is metabolism? Metabolism is a chemical reaction that occurs in the body’s cells, releasing energy from the nutrients ingested. Metabolism also uses energy to create other biologically important substances such as proteins. Basal metabolic rate (BMR) is a measurement of the body’s required energy to keep functioning at rest (measured in calories). Exertion, stress, fear, and illness increase the body’s metabolic rate.

The thyroid has many other bodily functions. The thyroid helps regulate calcium levels in the body. The thyroid can increase the body’s temperature, thus burning more calories. This in turn increases the body’s appetite. The thyroid also promotes glucose catabolism. Catabolism is the break down of complex glucose forms into simpler, more usable forms for energy usage. This gland stimulates protein synthesis, increases lipolysis. Lipolysis is the hydrolysis of lipids (fats), in which the lipids are broken down into simpler or usable forms. The thyroid also promotes normal heart function, normal neural development in fetus and growing infants, and normal neural function in adults.   

The thyroid is influenced by hormones produced by the pituitary gland and the hypothalamus. The pituitary gland is located at the base of the brain. This gland produces thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland to absorb iodine and then synthesize and release thyroid hormones.

The hypothalamus is located above the pituitary gland in the brain. This hormone produces thyrotropin releasing hormone (TRH). The hypothalamus and pituitary gland detect low levels of thyroid hormones in the blood. TRH is released by the hypothalamus to stimulate the pituitary gland to release TSH. TSH in turn stimulates the thyroid gland to produce more T3 and T4 hormones. This returns the thyroid hormone levels in the blood back to normal.

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