We evaluated serum angiotensin-converting enzyme levels as an index of thyroid hormone action in severe systemic illness. Angiotensin-converting enzyme was elevated in hyperthyroid patients (37.3 +/- 3.2 U/mL, p less than 0.01) and depressed in hypothyroid patients (17.9 +/- 1.4 U/mL, p less than 0.05). Compared with normal controls (22.6 +/- 1.6 U/mL), patients in an intensive care unit with free thyroxine index greater than 5.0 had normal angiotensin-converting enzyme levels (19.1 +/- 2.5 U/mL), but patients with a free thyroxine index less than 5.0 had angiotensin-converting enzyme levels significantly lower than the normal and hypothyroid groups (10.7 +/- 1.0 U/mL, p less than 0.05). A group of patients in an intensive care unit with alcoholic liver disease (known to elevate angiotensin-converting enzyme) and a low free thyroxine index had depressed angiotensin-converting enzyme levels (16.9 +/- 1.5 U/mL, p less than 0.01). A strong correlation was seen in the combined groups of all patients studied between levels of angiotensin-converting enzyme and a free thyroxine index (r = 0.70, p less than 0.01) and free T3 index (r = 0.72, p less than 0.01).