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How the thyroid works

MagazineJanuary 2002 (Vol. 12 Issue 10)How the thyroid works

The complex process of thyroid functioning is initiated in the hypothalamus and controlled by the pituitary gland

The complex process of thyroid functioning is initiated in the hypothalamus and controlled by the pituitary gland. Both are 'master glands', which exert control over not only the thyroid gland, but also the adrenal gland, the ovaries and testes. Furthermore, the hypothalamic-pituitary axis (HPA) integrates the central nervous and endocrine systems.

The thyroid gland is the largest endocrine gland in the body. It secretes thyroid hormones, which the body requires for growth and metabolism. Most of these thyroid hormones are T4 (L-tetraiodothyronine). A small amount of T3 (triiodothyronine) is secreted by the thyroid, but most is produced by T4 after it has left the thyroid gland. T3 is much more potent than T4.

The thyroid gland requires iodine to produce these hormones, which are synthesised in two steps. Two more processes are needed before the 'inactive' T4 can be stored. About 70 per cent of thyroid hormones are carried by thyroid binding globulin (TBG) in blood.

Many things can go wrong in these processes. When they do, the result is usually one of two conditions: hypothyroidism (abnormally low levels of thyroid hormones) or hyperthyroidism (too many thyroid hormones in the blood).

The principal drug treatment of hypothyroidism is the synthetic thyroxine (Eltroxin in the UK) or liothyronine, but thyroxine brings many problems of its own.

Hyperthyroidism can be treated by antithyroid drugs, radiolabelled iodine and surgery. The most commonly used antithyroid drug is carbimazole (Neo-Mercazole); others are propanolol, propylthiouracil, dibromotyrosine, dilodotyrosine, fluorotyrosine, iodine, or potassium or sodium iodine.

None of these drugs is without possible serious side-effects, especially bone marrow suppression leading to decreased blood leucocytes, as well as skin rashes, bronchitis, painful joints, hepatitis, jaundice and systemic lupus erythematosus-like syndromes.

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