High incidence of decreased serum triiodothyronine concentration in patients with nonthyroidal disease.

Serum thyroxine (T4) and triiodothyronine (T3) concentration and binding were measured in 34 clinically euthyroid patients hospitalized for a wide variety of nonthyroidal diseases. Despite clinical euthyroidism, serum T3 was in the hypothyroid range (less than 90 ng/100 ml) in 24 of the 34 patients, and the mean serum T3 of this group, 78.4 +/- 38.3 (SD), was significantly decreased from that of control, 134.0 +/- 29.3 ng/200 ml. Mean serum T4 levels were essentially the same in both groups, 7.3 +/- 2.0 for sick patients and 7.2 +/- 1.0 mug/100 ml for the controls. Plasma binding of both T4 and T3 was decreased in the patient group to 69.9 and 78% of control values, respectively. In accord with previous studies, the mean free T4 index, proportional to free T4 concentration, was significantly increased to 10.0 +/- 4.1 in the patient group (control, 7.6 +/- 1.3). However, the mean free T3 index of the patient group, 92.9 +/- 38.4 remained decreased from that of control, 138.9 +/- 34.4. Of the 24 patients with decreased serum T3 (less than 90 ng/100 ml), low T3 levels could be attributed to decreased plasma binding in 8; in 5, serum T3 was within the normal range for their advanced age. Mean TSH was greater in the patient group 2.6 +/- 1.9, than in the controls, 1.9 +/- 1.1 muU/ml. Moreover, the TSH response to administered TRH was moderately exaggerated in 7 patients with low free T3 index compared to 7 patients with normal free T3 index. Although significant statistically, neither the basal nor TRH induced TSH levels were in the range generally found in primary hypothyroidism. The data suggest that the high incidence of low serum T3 (70%) and free T3 index (32%) in nonthyroidal disease may be related to the catabolic state that accompanies illness rather than to specific disease entities. At the present time, the use of serum T3 or free T3 measurements for the diagnosis of hypothyroidism does not appear justified in patients with nonthyroidal disease.