What is thyroid hormone?

Thyroid hormone is a chemical compound that is produced, stored, and secreted by the thyroid gland. Thyroid tissue is made up of specialized cells that are organized into spheres called follicles. These specialized cells are called follicular cells. In normally functioning thyroid tissue, follicular cells are stimulated by a hormone called TSH that is secreted by the pituitary gland. TSH is also known as thyroid stimulating hormone or thyrotropin. TSH circulating in the bloodstream binds to structures called TSH receptors located on the outer surfaces of follicular cells, stimulating them to take up iodine from the bloodstream. Iodine that is taken up by follicular cells is used to produce thyroid hormone.

There are two active forms of thyroid hormone. The major form of thyroid hormone produced by the thyroid gland is called T4, also known as thyroxine. The abbreviation T4 reflects the fact that each molecule of thyroxine contains four iodine atoms. The other form of thyroid hormone produced by the thyroid gland is called T3, also known as triiodothyronine. The abbreviation T3 reflects the fact that each molecule of triiodothyronine contains three iodine atoms. Thyroid hormone molecules produced by follicular cells are stored in a mixture of proteins called colloid that accumulates in the center of each follicle.

When TSH binds to TSH receptors, thyroid hormone released from colloid is secreted into the bloodstream. In the normal state, about 77% of the thyroid hormone secreted by the thyroid gland is in the form of T4. The remaining 23% is in the form of T3. Most of the T4 secreted into the bloodstream is converted into T3 by enzymes called deiodonases that are present in the liver, the kidneys, the brain, and in other tissues throughout the body. T3 is the most active form of thyroid hormone. It binds to structures called thyroid hormone receptors that are located in cells throughout the body.

What does it do?

Thyroid hormone is active in a number of different organs and tissues throughout the body. During pregnancy, thyroid hormone helps to regulate the development of the fetus' brain and central nervous system. During childhood and adolescence, thyroid hormone helps to regulate the growth of the bones and maturation through the different stages of puberty. In adults, thyroid hormone helps to regulate the body’s metabolism by influencing the rate of energy consumption. It helps to regulate the cardiovascular system by influencing the rate at which the heart beats, the force with which the heart muscle contracts, and the degree to which blood vessels constrict and relax. It also helps to regulate the function of the gastrointestinal tract, the reproductive system, the respiratory system, and the brain and central nervous system.