Pineal, thyroid, parathyroid and thymus

Pineal gland, on the upper side of the forebrain: it secretes melatonin, which controls the body's 24-hour (diurnal) rhythms, such as when we sleep and wake and changes in body temperature. It also influences metabolism, pigmentation, the menstrual cycle and the body's defences.

Thyroid gland: two lobes on either side of the trachea (windpipe), joined by a thin band, the isthmus. Its follicle cells make thyroxine (T₄) and triiodothyronine (T₃), which need iodine. They control the body's basal metabolic rate (BMR, how fast it uses energy at rest), the making of red blood cells, the metabolism of carbohydrates, proteins and fats, and the balance of water and salts.

Thyroid disorders:

• Too little thyroid hormone (hypothyroidism), often from iodine shortage in food: the gland swells as a lump in the neck, goitre. In a pregnant woman, it can harm the baby: cretinism (stunted growth, poor mental development, deafness and inability to speak). In women it can also make the menstrual cycle irregular.
• Too much (hyperthyroidism), for example from cancer or lumps (nodules) in the thyroid. One form is exophthalmic goitre (Graves' disease): an enlarged gland, bulging eyes, a high BMR and weight loss.

The thyroid also makes thyrocalcitonin (TCT), which lowers blood calcium. On the back of the thyroid are four tiny parathyroid glands, which make parathyroid hormone (PTH). PTH raises blood calcium (it is hypercalcemic) by releasing calcium from bone, making the kidneys reabsorb it, and helping the gut absorb it from food. So PTH and TCT work against each other to keep calcium steady.

Thymus, between the lungs, behind the breastbone: it secretes thymosins, which help T-lymphocytes mature (for cell-mediated immunity) and help produce antibodies (humoral immunity). The thymus shrinks in old age, so the immune response becomes weaker in older people.