The modern therapeutic and diagnostic algorithms allow timely detection of pituitary disorder to prescribe adequate treatment. Meanwhile, when interpreting the thyroid status, physicians need to take into account the extremely rare but the actually existing possibility of central thyrotoxicosis. The worldwide practice shows that diagnosis of this condition is rather challenging. We report a clinical case of a TSH-secreting pituitary adenoma in a 47-year-old female who received a long-term thyrostatic therapy for thyrotoxicosis. The patient was diagnosed with Graves’ disease; however, thyrotoxicosis was actually caused by TSH-oma. The key laboratory signs of central thyrotoxicosis included the combination of episodes of normal or elevated TSH level with the high or normal free T4 level. MRI showed a pituitary macroadenoma. The clinical manifestations of thyrotoxicosis made it possible to rule out the thyroid hormone resistance syndrome. The attempted therapy with octreotide eliminated the clinical and laboratory signs of thyrotoxicosis, so the conservative method was selected as first-line therapy. The features of clinical and laboratory signs, as well as the principles of differential diagnosis and modern methods for treating TSH-secreting pituitary adenomas are discussed.