The diagnostic dilemma of isolated hyperthyroxinemia in acute illness.

Eighteen patients with a variety of illnesses but no history of thyroid disease were shown to have elevated serum total and free thyroxine (T4) concentrations but low serum total triiodothyronine (T3) levels. Thyroid function tests were requested initially to excluded the possibility of hyperthyroidism in a setting of hypermetabolism. Clinical and laboratory follow-up investigations demonstrated that 15 patients were euthyroid and that hormonal alterations were transient features of the intercurrent illnesses. Three patients did, in fact, have hyperthyroidism. Retrospective analysis of diagnostic data demonstrated that of the initial thyroid function screening tests, serum total T3 concentration, the T3/TF4 ratio, and the indirectly measured free T3 (FT3) index were the most useful parameters in excluding hyperthyroidism. The FT3 index was the most helpful, being normal or low in all the euthyroid and high in each of the hyperthyroid patients.