Angiotensin-converting enzyme activity. A potential marker of tissue hypothyroidism in critical illness

We measured serum angiotensin-converting enzyme (ACE) activity radiometrically as a possible indicator of reduced thyroid function in 57 euthyroid controls, 27 patients in a noncardiac intensive care unit (13 with medical and 14 with surgical disorders), and 29 patients having coronary artery bypass grafting. In the last group, blood was obtained preoperatively and one day and one month after surgery (group 1; n=18) or preoperatively and six hours and one day after surgery (group 2; n=11). Patients in group 1 had significant reductions in levels of serum thyroxine (T₄), triiodothyronine (T₃), and thyrotropin response to protirelin one day postoperatively. The ACE activity fell significantly. Patients in group 2 had low levels of T₄, T₃, thyrotropin, and ACE six hours postoperatively. All these levels remained low the next day, and free T₄ and free T₃ levels were also reduced; the reverse T₃ level became elevated. Changes in ACE significantly paralleled changes in T₃. The 27 patients without coronary artery bypass grafting also had significant reductions in serum T₄, T₃, and ACE levels. Dilution studies and dialysis of serum with low ACE activity failed to demonstrate an inhibitor to explain the reduced enzyme function.