The thyroid is a small gland located in the front of the neck, normally weighing less than one ounce. It is made up of two halves, called lobes, that lie alongside the windpipe (trachea) and are joined together by a narrow band of thyroid tissue, known as the isthmus. The thyroid is situated just below the "Adam's apple" or larynx.
The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyroid hormones: thyroxine (T4) and triiodothyronine (T3). Thyroid cells are the only cells in the body that can absorb iodine. These cells combine iodine and the amino acid tyrosine to make T3 and T4. T3 and T4 are then released into the blood stream and are transported throughout the body where they control metabolism (conversion of oxygen and calories to energy). Every cell in the body depends upon thyroid hormones for regulation of their metabolism. The normal thyroid gland produces about 80% T4 and about 20% T3, however, T3 possesses about four times the hormone "strength" as T4.
The pituitary gland controls the thyroid. This gland is about the size of a peanut and is situated at the base of the brain. When the level of thyroid hormones (T4 & T3) drops too low, the pituitary gland produces Thyroid Stimulating Hormone (TSH) that stimulates the thyroid gland to produce more hormones. Under the influence of TSH, the thyroid will manufacture and secrete T4 and T3 thereby raising their levels in the blood. The pituitary senses this and responds by decreasing its TSH production. One can imagine the thyroid gland as a central heating boiler and the pituitary gland as the thermostat. Thyroid hormones are like heat. When the heat gets back to the thermostat, it turns it off. As the room cools (the thyroid hormone levels drop), the thermostat turns back on again (TSH increases) and the boiler produces more heat (thyroid hormones).
Another gland, called the hypothalamus regulates the pituitary gland. The hypothalamus is part of the brain and produces TSH Releasing Hormone (TRH) and this tells the pituitary gland to stimulate the thyroid gland into releasing TSH. One might imagine the hypothalamus as the person who regulates the thermostat since it tells the pituitary gland at what level the thyroid should be set.
| System or Event Affected | Actions of T4/T3 |
| Basal Metabolism | Increases basal metabolic rate |
| Increases body temperature | |
| Increases appetite | |
| Carbohydrate, Lipid and Protein Metabolism |
Promotes glucose catabolism for energy |
| Stimulates protein synthesis | |
| Increases lipolysis | |
| Enhances cholesterol excretion in bile | |
| Heart | Promotes normal cardiac function |
| Nervous System | Promotes normal neuronal development in foetus and infant |
| Promotes normal neuronal function in adult | |
| Enhances effects of sympathetic nervous system | |
| Musculoskeletal | Promotes normal body growth and maturation of skeleton |
| Promotes normal function and development of muscles | |
| Reproductive | Promotes normal female reproductive ability and lactation |