TSH-secreting pituitary adenomas (TSHomas) are a very rare cause of central hyperthyroidism, accounting for about 2% of all pituitary adenomas. Diagnosis is often challenging as biochemical findings may be indistinguishable from thyroid hormone resistance or masked by coexisting autoimmune thyroid disease. The coexistence of a TSHoma and Graves’ disease is exceedingly rare and may produce complex and evolving patterns of thyroid function. We report the case of a TSH-secreting pituitary microadenoma which had presented with stable biochemical and radiologic findings over several years but was subsequently complicated by the development of Graves’ disease, leading to an abrupt presentation of thyrotoxicosis.

Clinical history and serial thyroid function tests, thyroid autoantibody profiles, pituitary MRI findings, and dynamic TRH stimulation testing were reviewed. Additional thyroid ultrasonography with Doppler vascularity assessment was performed. Several years of longitudinal follow-up data were analyzed to assess disease evolution, biochemical patterns, and therapeutic response. The patient underwent repeated laboratory monitoring every few months and interval pituitary MRI.

A woman in her mid-40s presented with long-standing fluctuating free thyroxine levels and inappropriately normal TSH concentrations. MRI revealed a pituitary microadenoma approximately 4 mm in size. TRH stimulation testing showed a blunted TSH response, supporting central hyperthyroidism due to TSHoma. Thyroid autoantibodies were initially positive for anti-TPO and anti-Tg, with negative TBII. The patient declined surgery and somatostatin analog therapy. For several years, her thyroid function and tumor size remained stable. During follow-up, she developed recurring palpitations and overt thyrotoxicosis; her free T4 was remarkably elevated while TSH remained unsuppressed. Repeated testing revealed new TBII positivity, and a Doppler ultrasound disclosed diffusely enhanced thyroid vascularity. The diagnosis of Graves' disease superimposed upon TSHoma was established. Carbimazole and beta-blockade improved the symptoms, normalized free T4, and slightly increased TSH. A repeated MRI scan showed the pituitary lesion to be unchanged.

This case shows the diagnostic challenge that is present when TSHoma with central hyperthyroidism is complicated by the new onset of autoimmune hyperthyroidism. The transition from a stable central hyperthyroidism to acute antibody-positive thyrotoxicosis emphasizes the role of longitudinal assessment, detailed autoimmune investigation, and selected application of TRH stimulation testing. Clinicians should consider the possibility of dual pathology when TSH and thyroid hormone levels do not exhibit expected feedback relationships, especially in the context of changing autoimmune status over time.