Physiology of Hormones

Hormones are produced by the cells of the endocrine glands. These glands are ductless and secrete the hormones directly into the bloodstream, which carries them to their sites of action. Hormones act as regulators of various bodily functions including the release of other hormones.

Synthesis of hormones

Hormone synthesis is closely controlled by an in situ homeostatic mechanism that prevents the over or under production of the various hormones.

Most hormones are regulated by a negative feedback system. This negative feedback prevents excess secretion of a hormone. For example, an excess of thyroid hormone in the blood signals to the hypothalamus and anterior pituitary gland to stop their respective release of thyrotropin releasing hormone (TRH) and thyroid stimulating hormone (TSH).

TSH stimulates the thyroid gland to secrete thyroxine. This inhibition of TSH release is an example of hormone self-regulation and is also called feedback inhibition. Other hormones such as estrogen, progesterone, testosterone and cortisol are also regulated by negative feedback inhibition.

Factors influencing the regulation of hormones include:

• Secretion of other hormones
• Plasma concentration of ions, chemicals or nutrients. For example, a high blood sugar level triggers the release of insulin from the pancreatic beta cells
• Environmental triggers such as light and dark cycles that stimulate or inhibit the release of melatonin from the pineal gland
• Nerve stimulation such as the release of stress hormones

Most hormones are synthesized as pre-hormones or prohormones. These are converted to their active forms before they reach their target cells.

Action of hormones

Hormones interact with their target receptors which are presented on the surface of or inside cells. Once activated, the receptors activate signal transduction pathways to bring about their actions. Steroid hormones and thyroid hormones work differently from peptide and protein hormones, which target receptors located in the cell membrane.

The steroid receptors are inside the cell, so the steroid needs to enter the cell by crossing the cell membrane. Once the steroid has bound to its receptor, the steroid-receptor complex moves into the nucleus of the cell where it binds to specific DNA sequences. This can either stimulate or suppress certain genes, to bring about the required action.