noun, plural: thyroid hormones
In animals, hormones are substances produced and secreted by an endocrine gland, the ductless gland of the endocrine system. In humans, one of the endocrine glands is the thyroid gland. The follicular cells (thyrocytes) of the thyroid gland are the cells that produce and secrete thyroid hormones. The cells make use of iodine to biosynthesize thyroid hormones. In humans, the two major thyroid hormones are the thyroxine and the triiodothyronine. The thyroid hormones are released into the bloodstream. There are more thyroxines in the blood is higher than triiodothyronines. And most thyroxines are bound to proteins, e.g. thyroxine-binding globulin, transthyretin, albumin, etc.
Both thyroxine and triiodothyronine are tyrosine-based and have iodine in their structure. A deficiency of iodine could impede the production of these hormones. This, as a result, could lead to a disease such as simple goiter.
The thyroid hormones are involved in the regulation of metabolism, and therefore, in increasing basal metabolic rate. They are involved in the regulation of protein, fat, and carbohydrate metabolism. They are also involved in stimulating vitamin metabolism. In other animals, such as mammals, they are involved in hibernation cycles. In birds, they are associated with the moulting behavior. In amphibians, they play a role in metamorphosis.
The target cells of the thyroid hormones are nearly all cells of the body. They cross the cell membrane via ATP-dependent carrier-mediated transport. Inside the cell, the thyroxine is converted into triiodothyronine through the action of the enzyme, deiodinase. The triiodothyronine reacts to the nuclear receptor, thyroid hormone receptor. This leads to the recruitment of the coactivator proteins and RNA polymerase, and the subsequent activation of gene transcription.
The thyroid-stimulating hormone (TSH) targets the pituitary gland to release thyroxine. Negative feedback occurs when thyroxine levels in the body are high, as this inhibits the secretion of TSH.
The hormone calcitonin which has hypocalcaemic effects is also of thyroid origin but is not usually classed with thyroxine and tri iodothyronine as a thyroid hormone.