The endocrine system consists of those glands which release their secretions into the blood stream so that, in small concentrations, they may exert physiological effects on target tissues which may be quite distant. A number of major endocrine glands are regulated more or less directly by the hypothalamus and pituitary gland. The pituitary gland (figure 1)may be thought of as being composed of two regions: an anterior lobe and a posterior lobe. The posterior lobe releases antidiuretic hormone, ADH (also known as vasopressin) and oxytocin. However, these two hormones are actually produced by neurons in the hypothalamus which project into the posterior pituitary. The anterior pituitary produces and releases hormones in response to releasing hormones (liberins) and inhibitory hormones (statins) from the hypothalamus.

The major hormones derived from the anterior pituitary are growth hormone/somatotropin (STH), which stimulates growth, prolactin, which stimulates lactation, thyrotropin, which stimulates the thyroid gland, adrenocorticotropic hormone(ACTH), which stimulates the adrenal cortex, follicle stimulating hormone (FSH) and luteinizing hormone (LH), which are involved in the development of the ovum and ovulation, and melanocyte stimulating hormone (MSH). Generally, anterior pituitary hormones are actually released as large and inactive precursors which are then cleaved into smaller, biologically active units.

The thyroid gland (figure 2) consists of two lobes which are anterolateral to the larynx, plus a connecting pyramidal lobe which is anterior to the larynx. The thyroid gland produces triiodothyronine (T3), thyroxin (T4) and calcitonin. T3 and T4 both require iodine for their synthesis and act to promote the metabolic rate of the body. Calcitonin tends to decrease the concentration of calcium in the blood and so encourages the uptake of calcium by bone.The thyroid gland has a rich blood supply that permits its important hormones to be rapidly secreted into the blood stream.

Embedded in the back of the lateral lobes of the thyroid gland are four small parathyroid glands (figure 3). The parathyroid glands produce parathormone, which encourages the movement of calcium into the bloodstream.

The adrenal glands (figure 4) are composed of two regions, the outer adrenal cortex, which is made up of 3 zones, and the inner adrenal medulla. The medulla is innervated by and so directly controlled by the sympathetic nervous system. It secretes epinephrine (adrenaline) and norepinephrine (noradrenalin). The adrenal cortex secretes three types of hormones: aldosterone, cortisol and androgens.

The other major endocrine glands are the pancreas (figure 5), dealt with in a previous lesson, and the ovaries (figure 6) and testes (figure 7) which will be discussed in other lessons related to the male and female reproductive systems. You will recall that the pancreas produces three important hormones: insulin, glucagon and somatostatin. The ovaries and testes produce a variety of hormones related to fertility and development.