Pineal and thyroid glands

The pineal gland lies on the dorsal side of the forebrain and secretes melatonin.

Melatonin is central to the body's 24-hour (diurnal) rhythm, for example the normal rhythms of the sleep-wake cycle and body temperature. It also influences metabolism, pigmentation, the menstrual cycle and the body's defence capability.

The thyroid has two lobes, one on either side of the trachea, joined by a thin flap of connective tissue, the isthmus. It is made of follicles and stromal tissue; each follicle is a ring of follicular cells around a cavity.

Follicular cells make two hormones: tetraiodothyronine or thyroxine (T₄) and triiodothyronine (T₃). Iodine is essential for making them at the normal rate.

Too little iodine in the diet causes hypothyroidism and an enlarged thyroid, called goitre.

Hypothyroidism during pregnancy disturbs the development and maturation of the growing baby, causing cretinism: stunted growth, mental retardation, low intelligence quotient, abnormal skin, deaf-mutism and so on. In adult women, hypothyroidism may make the menstrual cycle irregular.

Cancer of the thyroid or nodules in it can push hormone synthesis and secretion to abnormally high levels: hyperthyroidism, which harms body physiology. Exophthalmic goitre, also called Graves' disease, is a form of hyperthyroidism with an enlarged thyroid, protruding eyeballs, raised basal metabolic rate and weight loss.

Thyroid hormones regulate the basal metabolic rate, support red blood cell formation, control the metabolism of carbohydrates, proteins and fats, and influence water and electrolyte balance.

The thyroid also secretes a protein hormone, thyrocalcitonin (TCT), which regulates blood calcium by lowering it.