Thyroid growth immunoglobulins in feline hyperthyroidism

Feline hyperthyroidism bears a strong clinical and pathologic resemblance to toxic nodular goiter in humans. To evaluate whether the observed thyroid growth might be due to circulating thyroid antibodies, as has been postulated in humans, we studied the effect of purified immunoglobulin (Ig) G preparations on a rat thyroid follicular (FRTL-5) cell line. When compared with control, hyperthyroid cat IgG caused significantly increased [3H]-thymidine (Tdr) incorporation into DNA (p less than 0.02) and stimulated cellular proliferation 15-fold. Stimulation of 3H-Tdr incorporation tended to be biphasic and could be inhibited completely by a potent, specific TSH receptor blocking antibody. Hyperthyroid cat IgG also significantly inhibited 125I-bTSH binding to porcine thyroid membranes, an effect that could be reproduced using electrophoretically pure IgG and normal cat thyroid membranes. Unlike its effect on growth, hyperthyroid cat IgG did not stimulate intracellular cAMP, and there was no correlation between thyroid function in vivo and IgG growth-promoting activity in vitro. These data suggest that elevated titers of thyroid growth IgGs, probably acting through the TSH receptor, are present in feline hyperthyroidism and may play a role in goiter formation. Unlike growth, the thyroid hyperfunction observed is not IgG dependent. Further study of feline hyperthyroidism may contribute important insights into human nodular goiter and into the mediation of thyroid growth in general.