gamma-Glutamyl transpeptidase levels in thyroid disease.

To evaluate the association between serum, gamma-glutamyl transpeptidase activity (GGT) and thyroid function, we measured the GGT, alkaline phosphatase, free thyroxine index (FT4I), free triiodothyronine index (FT3I), and thyrotropin (TSH) levels in patients with hyperthyroidism and hypothyroidism and in normal subjects. Ten of 16 hyperthyroid patients had elevations of GGT activity. There was no difference in mean FT4I or FT3I between the groups with high GGT and normal GGT levels. Six months after treatment with radioactive iodine, serum GGT levels decreased, while there was no significant change in mean serum alkaline phosphatase levels. Mean serum GGT levels were decreased in patients with hypothyroidism and correlated well with serum TSH levels. Replacement therapy with levothyroxine sodium caused an elevation in mean serum GGT levels in six hypothyroid patients. Thus, serum GGT activity is frequently increased in patients with hyperthyroidism and may be decreased in patients with hypothyroidism. Euthyroidism results in restoration of normal GGT levels.