Effect of L-thyroxine on serum angiotensin converting enzyme activity in sheep

The effect of L-thyroxine on serum angiotensin converting enzyme activity using a sheep model was examined. Following two weeks of L-thyroxine treatment, the activity of serum angiotensin converting enzyme was increased from 6.92 U/ml to 8.65 U/ml (s.e.m. = 0.40, n = 13, P less than 0.01). Discontinuation of L-thyroxine treatment resulted in lowering the activity of serum angiotensin converting enzyme within three weeks to values close to those observed during the control period. It was concluded that L-thyroxine modulates serum angiotensin converting enzyme activity. The sheep is an appropriate animal model for the study of the factors that control serum angiotensin converting enzyme activity.