The Thyroid Accumulation, Oxidation and Metabolic Fate of 35S—Methimazole in the Rat

35S—Methimazole (MMI) was given ip to rats and the tissue distribution of radioactivity, thyroid metabolism and excretion studied over a period of 4 days. An accumulation of radioactivity and of unmetabolised methimazole was found in the thyroid, whereas the radioactivity disappeared rapidly in all other tissues studied. A tissue/plasma (T/P) ratio of radioactivity of 88.6 was demonstrated in the thyroid 12 hr after administration of a single dose of 35S—MMI with a T/P ratio of 2.1 and 1.7 in kidney and liver, respectively. All other tissues had a T/P ratio of less than one. TLC and liquid scintillation analysis of plasma and of thyroid homogenates showed a T/P ratio for 35S—MMI of 2.0 at 1 hr after the dose, rising to 25.5 after 8 hr. The methimazole was mainly oxidised to sulphate in the thyroid. TLC of plasma showed the presence of three compounds; methimazole, sulphate and protein bound radioactivity. Most of the radioactivity was excreted by the kidneys (the majority in the first 24 hr) with only 7.9...