Hormones are the body’s chemical messengers. They are very powerful, with only a tiny quantity needed to exert a big change. Some hormones can affect several parts of the body at once, for example, during puberty in boys, testosterone causes the voice to deepen, testicles and penis to grow, and muscles and body hair to develop. Others are more targeted in their action, such as thyroid-stimulating hormone, which has the sole function of controlling the amount of thyroid hormones released by the thyroid gland.

The hypothalamus in the brain connects the endocrine and nervous systems by releasing hormones that stimulate the pituitary gland. 

A very important role is the hypothalamus’s regulation of the levels of hormones that are in the bloodstream – a feat achieved via a phenomenon known as negative feedback inhibition. For example, the hypothalamus releases thyrotropin-releasing hormone (TRH), which tells the pituitary gland to secrete thyroid-stimulating hormone (TSH). In turn, this triggers the thyroid gland to release thyroid hormones. Once the levels of these hormones reach a threshold level in the bloodstream, negative signals are transmitted to the hypothalamus to stop the production of TRH and hence TSH and thyroid hormones. Once the levels of these in the bloodstream have dipped below the threshold, a positive signal goes to the hypothalamus causing TRH to be released, and so on, in order to maintain a steady state within the body. This way of working, with several endocrine glands combining to regulate hormone levels, is common and known as an endocrine axis.

The hypothalamus

The hypothalamus is key to homeostasis – the process that keeps the body functioning as it should. This part of the brain responds to all kinds of stimuli, from light (to regulate the 24 hour wake-sleep cycle) and smells, to stress and nerve impulses from various organs and systems, as well as, of course, hormones. It also controls emotions and behaviour, such as anger and libido.