A Rare Case of Thyrotropin Secreting Pituitary Macroadenoma Primarily Treated With Somatostatin Analogue

Abstract Introduction/Background: Thyrotropin secreting pituitary adenomas (TSH-oma) are a rare cause of hyperthyroidism. They account for <1% of the cases of hyperthyroidism with a reported incidence of 2.8 per 1 million in Sweden. Diagnosis is suspected by the presence of elevated T4 and T3 in the setting of an unsuppressed TSH level. The presence of large pituitary adenoma is highly suggestive of the diagnosis and can be differentiated from thyroid hormone resistance by elevated alpha subunit and SHBG levels. Trans-sphenoidal surgery is the definitive treatment. Peri-operative medical treatment with somatostatin analogues is indicated to achieve euthyroidism and prevent surgical risks and thyroid storm. The use of somatostatin analogues as a primary treatment for TSH-oma is still under investigation. We hereby report a rare case of TSH-oma where somatostatin analogues successfully resulted in normalization of thyroid function and tumor size reduction. Clinical Case: A 61 years old gentleman with a history of hypothyroidism diagnosed three years before presentation to the Pituitary clinic. He was treated with Levothyroxine. On clinical examination, he had mild tremor and warm sweaty palms with no stigmata of Grave’s disease. The thyroid function test showed elevated free T4 of 3.6 ng/dl (0.9-1.7), elevated free T3 of 8.6 pg/ml (2.0-4.4), and a high TSH level of 9.10 μIU/ml (0.27-4.20). His prolactin level was mildly elevated at 24.8 ng/ml(4.0-15.2). Testosterone, IGF-1, and cortisol levels were normal. An MRI of his pituitary gland showed large pituitary macroadenoma with supra-sellar extension and mild compression of the optic nerve. He had an elevated alpha subunit of 5.6 ng/ml (<1.37) and a high SHBG level of 198 nmol/l(10-80). TSH adenoma was diagnosed and he was planned for trans-sphenoidal surgery. Pre-operative treatment with somatostatin analogue Lanerotide 90 mg monthly injection was initiated. Interestingly normal thyroid function was observed approximately 1 month after his first injection. Repeat MRI showed a considerable decrease in the size of the pituitary macroadenoma. The patient opted to hold on to surgery and to continue on medical treatment. His thyroid function remains normal 15 months after initiation of treatment and his MRI continues to show stable pituitary adenoma. Conclusion: Somatostatin analogues can be used as a primary treatment for thyrotropin secreting pituitary adenomas when the patient is unable or unwilling to undergo surgery. It is use is associated with normalization of thyroid function and in some cases with a reduction in the adenoma size.