Thyroid hormones and functions

The thyroid gland wraps around the front of the windpipe(trachea) and expands into a lobe on either side . The thyroid gland produces the hormones thyroxineand calcitonin. It contains many round structures, called follicles, that produce, store, and release thyroxine. Cells in the spaces between the follicles produce calcitonin.Thyroxine is synthesized from two molecules of tyrosine,which then have four atoms of iodine chemically bonded to them. Thus, the thyroxine molecule is also called T4
The follicles also produce and release triiodothyronine, a version of thyroxine that has only three atoms of iodine and is called T3

 

The thyroid usually releases about four times as much T4 as T3. T3 is the more active hormone in the cells of the body, but when T4 is in circulation, it can be converted to T3 by an enzyme.

Thyroxine in mammals plays many roles in regulating cell metabolism. It does so by stimulating the transcription of a large number of genes in just about all cells of the body. These genes include those for enzymes in energy pathways, transport proteins, and structural proteins. As a result, thyroxine elevates the metabolic rates of most cells and tissues.

 It also promotes the use of carbohydrates rather than fats for fuel. Exposure to cold for several days leads to an increased release of thyroxine, an increased conversion of T4 to T3, and therefore an increase in basal metabolic rate. Thyroxine is especially crucial during development and growth, as it promotes amino acid uptake and protein synthesis by cells. Insufficient thyroxine in a human fetus or growing child greatly
retards physical and mental growth, resulting in a condition known as cretinism.

These hormones also support the process of red blood cell formation. Thyroid hormones control the metabolism of carbohydrates, proteins and fats. 

Maintenance of water and electrolyte balance is also influenced by thyroid hormones. Thyroid gland also secretes a protein hormone called thyrocalcitonin (TCT) which regulates the blood calcium levels.