The thyroid gland is under the control of thyroid-stimulating hormone (TSH) from the pituitary. It secretes thyroxine (T4) and triiodothyronine (T3). Iodine is essential for the synthesis of thyroid hormones. T4is probably converted to T3in peripheral tissues. Thyroid hormones have a role in growth and development, but their principal effect is the control of basal metabolic rate. Deficiency or excess affects all the tissues of the body, reducing or increasing the metabolic rate, resulting in hypothermia or hyperthermia, respectively. Deficiency during development produces mental retardation. Lack of iodine leads to thyroid swelling (goitre) caused by continuing stimulation by TSH. Calcium is one of the most tightly controlled ions in the body; abnormalities can produce muscle paralysis. Bone is the major store of calcium. Calcium reabsorption by the kidney is controlled by parathyroid hormone (PTH) produced by the parathyroid glands, which lie in the posterior part of the lobes of the thyroid gland. PTH secretion is controlled by blood calcium concentrations. Phosphate metabolism is intimately bound up with the control of calcium levels, as is the metabolism of vitamin D, which stimulates the absorption of calcium from the gastrointestinal tract and, in part, from the kidney.
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