Muscle protein breakdown in thyrotoxicosis assessed by urinary 3-methylhistidine excretion.

Urinary 3-methylhistidine (3MH) excretion has been studied in 20 thyrotoxic patients before, during and after treatment. 3MH excretion was high on diagnosis and fell significantly, when the patients had been rendered euthyroid, to levels measured in a control group. There was a significant linear correlation between 3MH excretion and free thyroxine index. The calculated muscle protein breakdown and 3MH/creatinine molar ratio also fell significantly after treatment of thyrotoxicosis.