Thyroid hormone is a derivative of the amino acid tyrosine. It has iodine atoms attached to it. Therefore, its biosynthesis requires iodide ions that must be supplied in the diet in the form of tablets. Otherwise Goiter can result in individuals that are deprived of iodine in their diet. In the United States usually table salt is supported with a supplementary amount of iodine to ensure healthy function of the thyroid gland.
There are two types of thyroid hormones that are obtained from each other by biochemical conversions. These are called: thyroxine and tri-iodo-thyronine. In thyroxine there are four iodide atoms attached to the hormone while in tri-iodo-thyronine there are only three iodide atoms. Most of the thyroxine hormone in the cells is converted to tri-iodo-thyronine in a biochemical process which involves the removal of one iodine atom from thyroxine. Therefore, the most dominant form of the thyroid hormone is tri-iodo-thyronine.
Both of these forms of the thyroid hormone are lipid soluble. Therefore, they can penetrate the phospholipid bilayer of the cellular membrane readily. Their receptors are located inside the nucleus and they exert their effect by increasing the rate of proteins translation or the synthesis of proteins in the cell.
Thyroid hormone is secreted by cells of the thyroid gland in response to a stimulation that it receives from the thyroid stimulating hormone. Thyroid hormone secretion is under a positive feedback control from the thyroid stimulating hormone (TSH) of the pituitary gland. Namely, high concentration of TSH stimulates high secretion of thyroid hormone and low level of thyroid stimulating hormone leads to low secretion of thyroid hormone.
Usually in states of hyperthyroidism in which there is excessive secretion of the thyroid hormone, there is in the blood little of the thyroid stimulating hormone. Also in the case of hypothyroidism in which there is deficient secretion of the thyroid hormone to the blood circulation elevated level of TSH are observed in the blood of affected individuals. Both of these effects that are observed in hyperthyroidism and hypothyroidism are direct result to the negative feedback control of the thyroid hormone on TSH or thyroid stimulating hormone.
Thyroid hormone unlike other hormones in the body does not have a specific target organ to act upon. Instead it acts in a universal manner in which its effect is done on all the cells of the body. Its action increases all aspects of metabolism inside the body. In this sense, it has similarity to the hormones epinephrine and norepinephrine as far as the increase in metabolic rate in the body is concerned.
One of the effects of the thyroid hormone is on carbohydrates metabolism in the body. Thyroid hormone increases all aspects of sugar metabolism including the uptake of glucose by cells in the liver and it increases th rate at which glucose is synthesized by the process of gluconeogenesis. Also the thyroid hormone increases the rate at which compounds are absorbed in the gastrointestinal tract. All these effects that were mentioned earlier are probably directly related to the increased rate of enzymes synthesis by the effect of the thyroid hormone on all cells of the body.
In addition to its effect on carbohydrates metabolism thyroid hormone has an effect on lipids metabolism inside the body. Lipids, under the effect of the thyroid hormone are promptly mobilized from fats tissue and are readily metabolized in the liver. The thyroid hormone decreases the level of cholesterol in the blood. In addition, it decreases the level of the phospholipids also.
The increased rate of metabolic reactions in the body by the effect of the thyroid hormone makes increased requirement for cofactors and vitamins in the body. This is so due to the increased activity of enzymes in the body by the effect of the thyroid hormone. For example, the increase in glucose metabolism in the body due to the effect of thyroid hormone renders a need for more of the hormone insulin to speed up the uptake of glucose by the liver cells.