Background: Low free T4 and normal/low TSH concentrations are often noted in clinical practice and denote presence of “Euthyroid Sick Syndrome” or Central Hypothyroidism. However, accurate diagnosis is difficult even with determination of free T3 being low/normal in both. Repeated determination of these tests may help differentiate between these disorders. Objective: Evaluation of reverse T3 to differentiate between “Euthyroid Sick Syndrome” and Central Hypothyroidism. Subjects and Methods: Free T3 and Reverse T3 were determined as “add on” tests using previously drawn blood samples of 78 consecutive adults showing low free T4, 0.80 ± 0.02 and low/normal TSH, 1.29 ± 0.40 [normal ranges, 0.89 - 1.70 mcg/dl; 0.45 - 4.67 uU/ml]. Free T4, free T3, TSH and reverse T3 levels were also determined in age-matched 35 healthy volunteers and reassessed in study group. Statistical analyses for comparisons were conducted between groups. All data are reported as Mean ± SEM. Results: Reverse T3 established two distinct groups: 1) subnormal concentrations, 8.31 ± 0.52 [range, 11 - 14 ng/dl]; 2) supernormal levels; 32 ± 4 [normal Range 12 - 26]. Free T3 concentrations were subnormal or normal, 1.6 - 2.9 [normal range, 2.3 - 4.2 ng/ml] in individuals amongst both groups. On reassessment after 3 - 6 weeks, free T4, free T3, TSH and reverse T3 normalized in group with normal or elevated reverse T3 indicating recovery from “Euthyroid Sick Syndrome” whereas free T4 and reverse T3 remained subnormal in the other group suggesting presence of Central Hypothyroidism. Conclusion: Reverse T3 is a reliable laboratory test differentiating between Central Hypothyroidism and “Euthyroid Sick Syndrome” in subjects with low free T4 and low/normal TSH levels.
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