SUN-577 A Remarkable Case of Thyrotoxicosis Initially Caused by Graves' Disease Followed by a TSHoma

Background: Graves’ disease is the commonest cause of thyrotoxicosis while thyrotropin producing pituitary adenomas (TSHoma) are very rare and represent 2-3.5% of all pituitary adenomas. Coexistence of a TSHoma and Graves’ disease has been very rarely reported. We present a case of a lady with a TSHoma initially presenting with primary thyrotoxicosis, likely Graves’ Disease. Case: A sixty-eight year old lady was referred to our department with thyrotoxicosis, a fT4 of 20.4pmol/L (7.0-16.0) and a TSH of <0.02mIUL (0.50-4.20). A technetium pertechnetate scan was consistent with Graves’ Disease. She was treated with carbimazole for 18 months and remained clinically and biochemically euthyroid. After stopping carbimazole her fT4 started to rise but with normal TSH. This was confirmed on multiple platforms. A TRH stimulation test demonstrated a flat TSH response and a pituitary MRI showed pituitary microadenoma. Prolactin, IGF-1 and other pituitary profile were normal. She had a 11C-Metionine PET/CT and SPGR MRI scan which demonstrated high activity on the left lateral side of the pituitary fossa consistent with a functioning adenoma. She was treated with cabergoline and octreotide but failed to tolerate either. For now the patient has decided to manage this lesion conservatively. Conclusion: This is a very unusual case of thyrotoxicosis caused by two different processes in the same patient. This case highlights the need to reconsider the diagnosis of a TSHoma when faced with discordant thyroid function tests. It also highlights the utility of 11C-Methionine PET/CT scans in the diagnosis of functional endocrine tumours.