Thyroid status and muscle protein breakdown as assessed by urinary 3-methylhistidine excretion: study in thyrotoxic patients before and after treatment.

Urinary 3-methylhistidine (3MH) excretion was studied in nine thyrotoxic patients before and after treatment. Urinary creatinine (Cr) output was also measured and was low in the thyrotoxic subjects before treatment. Thus, although urinary output of 3MH was not greater than among the control population when expressed per subject, it was significantly elevated when expressed as the ratio of 3MH to Cr; this ratio fell significantly, reaching normal control values after a euthyroid state was obtained. In one patient who became hypothyroid, the 3MH/Cr ratio fell under the control value. There was a significant linear correlation between the 3MH/Cr ratio and the hormonal variables (T3, T4, FT4l); moreover, variations in the 3MH/Cr ratio and variations in the T3 level were closely correlated. 3-Methylhistidine appears to be a reliable index of muscular breakdown in thyrotoxicosis. From our results, it can be concluded, first of all, that hyperthyroidism is accompanied by an increased muscular catabolism, and, second, that the return to a euthyroid state results in an immediate normalization of muscular breakdown.