Reverse T-3 does not reliably differentiate hypothyroid sick syndrome from euthyroid sick syndrome

To assess the efficacy of reverse T-3 in differentiating between the hypothyroid and euthyroid state in the setting of illness, all reverse T-3 determinations obtained over a 4-year period in a University teaching hospital were analyzed in the context of concurrent thyroid function tests, bilirubin, albumin, creatinine, subsequent treatment, and follow-up, Based on T-4 (or free T-4 index) and TSH, the thyroidal state of the patient and the appropriateness of the reverse T-3 determination were assigned, A total of 262 reverse T-3 determinations were made in 246 patients, There is an inverse linear relationship between the log TSH and the reverse T-3. Patients with hypothyroidism plus illness may have a normal reverse T-3 and patients with euthyroidism may have a low reverse T-3 Reverse T-3 is linearly related to bilirubin up to a bilirubin of approximately 171 mu M (10 mg/dL). Sixty percent of the reverse T-3 determinations were obtained for seemingly inappropriate indications, In association with a low free T-4 index/T-4, an unmeasurable reverse T-3 did not lead to institution of thyroid hormone treatment in over 52% of cases, Although reverse T-3 may be elevated in the setting of nonthyroidal illness, it is not reliable in distinguishing between the hypothyroid sick patient and the euthyroid sick patient, This is probably because of drug and disease effects on thyroid hormone metabolism as well as the presence of sufficient T-4 substrate for conversion to reverse T-3 in many hypothyroid sick patients.