MODE OF ACTION OF CARBIMAZOLE IN GRAVES' DISEASE

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Abstract
SUMMARY: In a series of hyperthyroid subjects treated with a high dose ‘block‐replace’ regime, mean thyroidal technetium uptake was shown to fall progressively over an 18 month period. Furthermore, sub‐total inhibition of hormone biosynthesis could be demonstrated, without evidence of proportionate inhibition of organification, the regression of radio‐iodine on technetium thyroid uptake showing similar slopes in euthyroid, treated and untreated hyperthyroid subjects. Together with the reported differential in remission rates between propranolol and anti‐thyroid drugs, it is concluded that carbimazole does not act solely by inhibiting intrathyroid hormonogenesis, but more definitively by affecting thyroid hyperstimulation at a prebiosynthetic level.The inability to demonstrate normal suppressibility of trapping in any subject in the present series despite extended therapy with a ‘block‐replace’ regime, further suggests that current concepts of ‘suppressibility’ are invalid, the progressive fall in technetium uptake being unlikely to represent spontaneous restoration of a normal pituitary‐thyroid axis, but rather a direct influence of prolonged therapy with anti‐thyroid drugs on the natural history of the disease.