Primary hypothyroidism with grossly elevated plasma total thyroxine and triiodothyronine levels.

In an elderly patient with clinical primary hypothyroidism and a raised basal TSH, the serum free thyroxine (fT4), total thyroxine (TT4) and triiodothyronine (TT3) were consistently and paradoxically grossly elevated when measured by radioimmunoassay. In part these hormone levels were due to a high titre of endogenous IgG immunoglobulins which bound T4, T3 and reverse T3 (rT3) and thus caused gross interference in the radioimmunoassays. However, when this methodological interference was removed by using a methanolic extract of the patient's serum, the concentrations of TT4 and TT3 were still grossly elevated. It was only when basal TSH and the concentration of fT4 and fT3 were measured by equilibrium dialysis that these hormone levels were found to be consistent with primary hypothyroidism.