Thyroxine and triiodothyronine autoantibodies in Hashimoto's thyroiditis with severe hormone-resistant hypothyroidism.

Severe hypothyroidism in an 8 year-old girl was associated with a paradoxically high free thyroxine (T4), a high TSH level and antimicrosomal antibodies suggestive of Hashimoto's thyroiditis. Low radiolabelled T3 binding to resin in the standard T3 resin uptake test suggested thyroid hormone binding which was subsequently found to be due to antibodies to T4. T4 by equilibrium dialysis was very low confirming that conventional free T4 and total T4 assays overestimated the true values. Subsequent normalization of free T4 by dialysis coincided with a decline in the T4 autoantibody titer allowing a change in treatment from Cytomel (triiodothyronine) to Synthroid (L-thyroxine) while maintaining therapeutic efficacy.