Thyroid hormones act on the
kidney,
liver,
heart,
nervous system,
skeletal muscle and other tissues, making them more responsive to
epinephrine and stimulating
cellular respiration. Thyroid hormones act slowly and make take up to 48 hours after release to have any noticeable effect.
Functions of Thyroid Horomones
- thermoregulation - because thyroid hormones increase the rate of cellular respiration, a related effect is the generation of heat. As a result, many vertebrates use this gland to control thermoregulation (maintaining body temperature).
- salinity - in fish, the thyroid gland helps the organism adapt to the salinity of its environment. Secretions of the thyroid often specify the preference of a fish for salt or fresh water.
- growth and development - hypothyroidism, low levels of thyroid hormone production can result in underdeveloped body structures and very low metabolism. This disease is known as cretinism in humans.
- metamorphosis - the level of thyroid hormone production often correlates with the stage of metamorphosis.
thyroid hormone synthesis
Two major thyroid hormones are
thyroxine and
3,5,3-triiodothryonine (also known as T
4 and T
3). They are synthesized by the iodination of two
tyrosine amino acids.
Iodine is acquired from the blood and sequestered in the
thyroid gland.
Thyroid Stimulating Hormone (TSH) is synthesized in the adenohypophysis region of the brain and is responsible for stimulating thyroid hormone production in the thyroid gland. TSH is in turn upstream regulated by TSH Releasing Hormone (TRH) which is secreted by the hypothalamus and acts on the adenohypophysis. Negative feedback from thyroid hormones in the blood downregulates TRH production resulting in some degree of automatic maintenance of thyroid hormone levels in the blood. On top of this chemical regulation, the hypothalamus is able to control the amount of thyroid hormone in the body. Low skin temperature, for example, will trigger the hypothalamus to release TRH and thereby increase body temperature.