Unexpected clinical course during treatment of a TSH-secreting pituitary adenoma.

OBJECTIVE: To report the rare occurrence of a patient with thyrotropinoma that transitioned into a secretory thyro-somatotroph adenoma during medical treatment with somatostatin analogue.
METHODS: We report the case of a patient with a thyrotroph pituitary adenoma who developed de novo evidence of growth hormone cosecretion following one year of successful medical treatment.
RESULTS: A 78-year-old woman was diagnosed with a thyroid stimulating hormone (TSH) secreting pituitary macroadenoma (TSHoma) based on classical clinical and biochemical features. There was no clinical or biochemical evidence of growth hormone (GH) cosecretion. She declined surgical resection and was treated with primary medical therapy, octreotide long acting repeatable (LAR), to which she had an antitumor and antisecretory response; however, following 12 months of successful medical treatment she developed de novo hypersecretion of growth hormone despite involution of the tumor mass. TSH-secreting pituitary adenomas may rarely become plurihormonal during apparently successful medical treatment. This may represent an unusual form of secondary resistance to somatostatin analogue or the rarer phenomenon of tumor transformation into a secretory thyro-somatotroph adenoma.
CONCLUSION: The unexpected clinical course of this case highlights the need for careful long-term surveillance in patients with TSH secreting pituitary adenomas.