Serum angiotensin-converting enzyme. Alterations in hyperthyroidism, hypothyroidism, and subacute thyroiditis.

Angiotensin-converting enzyme (ACE) activity was measured in the serum samples of 247 patients with varying thyroid states. In 60 hyperthyroid patients, the mean (+/- SD) ACE level of 29.5 +/- 9.7 units/mL was higher than in all other groups. The serum ACE level was 17.0 +/- 5.1 units/mL in 129 euthyroid subjects and differed from the level of 13.9 +/- 5.1 units/mL observed in 34 hypothyroid patients. Twenty-four patients receiving exogenous thyroid hormone had elevated serum thyroxine values. Their mean serum triiodothyronine level was in the normal range, and the mean ACE level did not differ from the euthyroid mean. The mean serum ACE level fell from 30.8 to 17.4 units/mL in 35 hyperthyroid patients studied before and after therapy. In 12 hypothyroid subjects, the mean serum ACE level rose from 11.6 to 15.8 units/mL after thyroid hormone replacement. In eight of ten patients with transient hyperthyroidism (subacute thyroiditis or painless thyrotoxic thyroiditis), their highest ACE levels were observed in the hyperthyroid or transition phase and fell progressively with the lowest values being recorded during the hypothyroid or early recovery phases. Thus, ACE activity may respond to thyroid hormone, and interpretation of serum ACE levels may require knowledge of the patient's thyroid status. Serum ACE may be useful as a probe for exploring peripheral thyroid hormone action.