IgA deficiency during treatment of infantile hypothyroidism with thyroxine.

Serum IgA concentrations in five children with infantile hypothyroidism fell soon after the start of treatment with thyroxine. In one child the IgA concentration fell appreciably (to less than 0.01 g/1) and remained reduced; in the four others it returned to normal. IgM and IgG concentrations were roughly normal throughout. The deficiency in IgA concentrations may have been due to stimulation by thyroxine treatment of a T cell suppressor system that, in the original hypothyroid state, was less than normally active.