Increased levels of serum angiotensin-converting enzyme activity in hyperthyroidism.

Serum angiotensin-converting enzyme activity was significantly elevated in 21 patients with hyperthyroidism (65 +/- 18 U/mL) as compared with levels in healthy control subjects (30 +/- 9 U/mL). In eight patients treated with thionamide drugs, the enzyme activities decreased to a normal range as thyroid hormone concentrations returned to euthyroid levels. When two patients became hyperthyroid due to an insufficient maintenance dose of antithyroid drugs, the increase in the serum thyroid hormone concentration was followed by the re-elevation of serum angiotensin-converting enzyme levels with a time lag of about 2 weeks. Levels of this enzyme are increased reflecting the elevated serum thyroid hormone levels in hyperthyroidism. The hyperthyroid state should be considered when serum angiotensin-converting enzyme levels are high.