Abstract
A 35-year-old Asian male, treated for hyperthyroidism, systemic lupus erythematosis, and uremia presented with low serum total thyroxine (T4) and normal serum thyrotropin (TSH) levels. He had been receiving prednisone and methimazole for 15 weeks. Free T4 measured by direct equilibrium dialysis was in the hypothyroid range (0.3 ng/dL; normal, 0.8–2.7). Two possibilities were considered: (1) a weakly bound dialyzable inhibitor in uremic serum that interfered with this serum free T4 determination or (2) hypothyroidism with persistent TSH suppression because of prior hyperthyroidism. To determine whether a weakly bound inhibitor was involved, the patient's serum was serially diluted using two diluents: (1) an ultrafiltrate of the patient's serum, which would contain any unbound inhibitor, as well as free T4 and (2) an inert diluent. Free T4 measurements were similar with both, providing evidence against the presence of a dialyzable and ultrafilterable inhibitor. In conclusion, this patient was hypothyroid because of antithyroid drug administration, associated with prolonged central TSH suppression from preexisting hyperthyroidism. Discontinuation of methimazole resulted in normalization of serum total T4 and TSH values. Thus, paired, serial serum dilutions, using two different diluents, provided evidence for differentiation of appropriately low free T4 measurements (because of hypothyroidism), from spuriously low free T4 measurements (because of an interfering inhibitor).
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