ANTITHYROID AGENTS are used in the treatment of overactivity of the thyroid gland — hyperthyroidism. thyrotoxicosis or Graves’ disease. In thyrotoxicosis there is excess secretion of the thyroid hormones, thyroxine (T4) and triiodothyronine (T3; liothyronine). This excess results in an exaggerated version of the normal activity of the gland, so that there are the symptoms of increased metabolic rate, an increase in body temperature, sweating, increased sensitivity to heat, nervousness, tremor, raised heart rate, tendency to fatigue and sometimes loss of body weight with an increased appetite. The cause of thyrotoxicosis may be simple overactivity of the gland; or toxic nodular goitre where there is secretion from a benign tumour or a carcinoma of the thyroid; or diffuse toxic goitre (Graves’ disease; exothalmic goitre) in which there are additional symptoms, including a swelling of the neck (goitre) due to enlargement of the gland, and protrusion of the eyes (exothalmos). How the disease is treated depends on its origin, but one final therapy is surgical removal of part of the gland or, more commonly, treatment of the gland with radioactive iodine to reduce the number of cells. For this purpose 131I is given orally and emits 7-radiation, which has little effect, and 3-radiation, which is locally cytotoxic. Hypothyroidism will eventually be produced, but this can be treated (see thyroid HORMONES). Also, 131I can be used for diagnostic purposes. In any event, drugs are used, either to control the symptoms in the long term, or in the short term to prepare the gland for more radical intervention. B-Blockers are commonly used in the prevention of a number of the signs and symptoms of thyrotoxicosis, by blocking the effects of overstimulation of the release of adrenaline and noradrenaline by thyroid hormones. Beta-Blockers used include metoprolol, nadolol. propranolol and sotalol (see 8-adrenoceptor blockers). Some other drugs — chemically thionamides (thioureylenes), e.g. carbimazole, methimazole, propylthiouracil — act directly on the thyroid gland to reduce the production of the thyroid hormones, so treating the excess of thyroid hormones in the blood. Iodine itself (which is chemically incorporated into the thyroid hormones thyroxine and triiodothyronine), can be given (as aqueous iodine oral solution, or Lugol’s solution), to suppress gland activity prior to thyroid surgery.