A promising direction in the treatment of autoimmune hyperthyroidism is the development of low-molecular-weight antagonists of thyroid-stimulating hormone (TSH) receptor. The effect of the thieno[2,3-d]-pyrimidine derivative TPY1 on TSH-stimulated synthesis of thyroid hormones in the culture of FRTL-5 thyrocytes and on thyroliberin-stimulated production of thyroid hormones in rat blood was studied. Preincubation of FRTL-5 cells with TPY1 suppressed the stimulatory effect of TSH on the thyroxine and triiodothyronine synthesis. When administered to rats, TPY1 (i.p., 25 mg/kg) reduced thyroliberin-stimulated levels of thyroid hormones in the blood, and in the thyroid gland inhibited the gene expression of thyroid peroxidase, thyroglobulin, and Na+/I—-cotransporter responsible for thyroxine synthesis. In the absence of thyroliberin stimulation, TPY1 did not affect thyroid hormone levels and expression of thyroidogenesis genes. Thus, a new TPY1 antagonist of TSH receptor has been developed, which can become a prototype of a drug for the treatment of autoimmune hyperthyroidism.