Serum angiotensin-converting enzyme (ACE) activity varies directly with thyroid hormone levels in states of altered thyroid function. Because T3 levels decrease during fasting, ACE activity was examined to ascertain if it was reduced in this low T3 condition. Eighteen obese euthyroid subjects were hospitalized and placed on a weight-maintaining diet for 4 days. Nine subjects (Group 1) underwent a fast (50 kcal/day) for 8 days. Nine (Group 2) subjects received T3 (5 μg q 3 h) during an identical fast. Weight loss was identical in both groups (-4.4 kg). Serum T3 fell in Group 1 from 104 ± 8 to 50 ± 4 ng/d/(p<.05) but was unchanged in Group 2 (114 ± 11 ng/dl fed vs. 120 ± 14 ng/dl fasted). Blood pressures fell significantly in Group 1 (mean systolic: 112 → 104 mmHg; diastolic: 71 → 65 mmHg, p<0.05), but not in Group 2 subjects. ACE activity fell progressively in Group 1 subjects during fasting (14.4 ± 1.6 U/ml fed vs. 12.8 ± 1.4 U/ml fasted p<0.05). ACE activity was not decreased significantly early in the fast in patients given T3, but by late fast (days 6-8) was reduced to the same degree as in Group 1 subjects. Glucose and insulin levels fell similarly in both groups. Conclusions: (1) ACE activity is reduced during starvation. This effect is not mediated by T3. (2) Blood pressure reduction during fasting may result from the low T3 levels, but not from decreased ACE activity. Interpretation of serum ACE activity must be viewed in the context of a patient's diet.
ASJC Scopus subject areas
- Endocrinology, Diabetes and Metabolism
- Clinical Biochemistry
- Biochemistry, medical