| Abstract: | In order to ascertain whether angiotensin I converting enzyme (ACE) activity might be regulated by thyroid hormone, serum ACE activity was measured in a variety of thyroid states, including hyperthyroid and hypothyroid subjects. In addition, the correlation of serum ACE activity to plasma renin activity (PRA) and plasma aldosterone concentration (PAC) was evaluated in these patients. In hyperthyroid patients, the mean (+/- SD) serum ACE activity was 32.7 +/- 6.7 U/ml (n = 30), which was significantly higher than that in hypothyroid patients (20.4 +/- 4.3 U/ml, n = 7, p less than 0.001) and in normal subjects (22.5 +/- 3.4 U/ml, n = 51, p less than 0.001). No significant difference in serum ACE activity was found between the hypothyroid patients and normal subjects. There was a significant positive correlation between serum ACE activity and PRA (r = 0.524, n = 30, p less than 0.01) and also between serum ACE activity and PAC (r = 0.473, n = 30, p less than 0.01) in the patients with hyperthyroidism. By contrast, no significant relationship was observed between serum ACE activity and thyroid hormones (r = 0.115, for T3; r = 0.143, for T4) in hyperthyroid patients. Treatment with furosemide (1 mg/kg i.v.) and upright posture (2h) significantly increased PRA, PAC and serum ACE activity in both hyperthyroid patients and normal subjects, but not in hypothyroid patients. There was a significant positive correlation between changes in serum ACE activity and in PRA (r = 0.418, n = 23, p less than 0.05) in response to the treatment in hyperthyroid patients, while no significant relationship was observed between them in either hypothyroid patients (r = 0.216, n = 6, p less than 0.10) or normal subjects (r = 0.620, n = 10, 0.05 less than p less than 0.01). In one patient with hyperthyroidism, administration of propranolol decreased PRA from 3.4 to 2.3 ng/ml/h, corresponding to an apparent decrease in serum ACE activity from 38.7 to 29.6 U/ml. From these results, it is suggested that serum ACE activity in the hyperthyroid state is modulated by the renin-angiotensin system rather than by thyroid hormone. |
