Abstract. We studied the blocking type TSH receptor antibodies in 28 patients with primary myxedema and 21 patients with goitrous Hashimoto's thyroiditis by measuring the ability of their IgGs to inhibit TSH binding to its receptor, and to inhibit TSH-stimulated cAMP increase and [3H] thymidine incorporation in a rat thyroid cell line, FRTL-5. The incidences of TSH binding inhibitor immunoglobulin, thyroid stimulation inhibiting immunoglobulin and thyroid growth inhibiting immunoglobulin in patients with primary myxedema were 54.6, 75 and 65.2%, respectively, against 14.3,0 and 17.7%, respectively, in goitrous Hashimoto's thyroiditis. The antibodies inhibited dose-dependently not only TSH stimulated but also Graves' IgG-stimulated cAMP increase and [3H] thymidine incorporation. The TSH binding inhibitor immunoglobulin activities in patients with primary myxedema were significantly correlated with both the thyroid stimulation inhibiting immunoglobulin (r = 0.665; P<0.01) and the thyroid growth inhibiting immunoglobulin (r = 0.618; P<0.01) activity. Thirteen patients whose TSH binding inhibitor immunoglobulin activities were more than 50% had both strong thyroid stimulation inhibiting immunoglobulin (75.1–100%) and thyroid growth inhibiting immunoglobulin (57.4–100%) activities. These data suggest that the vast majority of patients with primary myxedema have potent blocking type TSH receptor antibodies. These might play a role in primary myxedema causing hypothyroidism and thyroid atrophy through inhibiting TSH-stimulated cAMP generation.

A 40-year-old male who developed Graves' hyperthyroidism following primary hypothyroidism is reported. He presented with clinical signs of hypothyroidism and concomitant myasthenia gravis. The thyroid was not palpable. He was treated with T4, pyridostigmine and prednisolone. One year later he developed hyperthyroidism and goitre. His initial serum IgG had no intrinsic thyroid stimulating activity, but showed almost complete inhibition of TSH-stimulated cAMP generation (99.4%, normal <38%) and [3H]thymidine incorporation (99.5%, normal<40%) into rat thyroid cells, FRTL-5 cells, with very high activity (80.2%, normal <15%) of TSH binding inhibitor immunoglobulin. When he developed hyperthyroidism and goitre, his IgG showed a strong thyroid stimulation, both cAMP production (27-fold increase) and [3H]thymidine incorporation (5.5-fold increase). No inhibitory activities were noted. These findings suggest that clinical states of autoimmune thyroid diseases can be changed in accordance with changes of functional properties of TSH receptor antibodies.