THYROXINE AND TRIIODOTHYRONINE AUTOANTIBODIES IN HASHIMOTO THYROIDITIS WITH SEVERE HORMONE-RESISTANT HYPOTHYROIDISM

Severe hypothyroidism in an 8 year-old girl was associated with a para doxically high free thyroxine (T4), a high TSH level and antimicrosoma l antibodies suggestive of Hashimoto's thyroiditis. Low radiolabelled T3 binding to resin in the standard T3 resin uptake test suggested thy roid hormone binding which was subsequently found to be due to antibod ies to T4. T4 by equilibrium dialysis was very low confirming that con ventional free T4 and total T4 assays overestimated the true values. S ubsequent normalization of free T4 by dialysis coincided with a declin e in the T4 autoantibody titer allowing a change in treatment from Cyt omel (triiodothyronine) to Synthroid (L-thyroxine) while maintaining t herapeutic efficacy.