Iodine, TSH and the size of the thyroid.

Hypothalamus → TSH → follicles make T3/T4 from iodine, which feeds back to shut TSH off; low iodine leaves TSH high and the gland swells into goitre, nodules overshoot into hyperthyroidism.

Bring T4 back into range for a goitrous patient using only diet, and explain why the gland enlarged when iodine was low.

Goitre means too much thyroid hormone.

• 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.

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