SUN-593 Puzzling TFTs: A Case of a TSHoma and Underlying Hashimoto's Disease

Introduction: TSH-secreting adenomas (also known as TSHomas or thyrotropinomas) are rare pituitary tumors (0.5-3% of pituitary adenomas) and a rare cause of hyperthyroidism (<1%). Characteristic lab values include a non-suppressed TSH with elevated T4 and T3. Clinical Case: A 61 year old man with chronic exhaustion was found to have an elevated TSH in 2013. Family history was notable for hypothyroidism in his sister and mother. His PCP started the patient on levothyroxine (LT4) and escalated dose up to 175 mcg daily. However, the TSH remained persistently elevated despite good adherence. His levothyroxine was increased to 215 mcg and he developed palpitations. TSH decreased from 11.89 (0.35-5.5 uIU/L) to 4.7 with this dose change, but free T4 increased from 1.61 to 2.54 (0.8-1.9 ng/ml). A pituitary MRI in 7/2015 showed a hypoenhancing 8x8 mm pituitary microadenoma. He was then seen for a second opinion, taken off levothyroxine and monitored. Physical exam was notable for slight tachycardia with HR 105, no thyromegaly or nodules, and otherwise the patient was euthyroid appearing. Labs were otherwise notable for an anti-TPO antibody 261 (0-115 IU/ml), TSI <89 (<140%), alpha subunit 1.6 (0.1-0.5 ng/ml), SHBG 76.4 (17-66 nmol/L), TSH 14.37, and free T4 2.3. Repeat MRI in 4/2018 demonstrated an interval increase of pituitary adenoma (1.3x1.2x1.1 cm). Methimazole was started (partially as a diagnostic test) and TSH increased from 44 to 134. The patient underwent uncomplicated transphenoidal surgery (TSS). Immunohistochemistry of the tumor demonstrated positive TSH staining. At 6 weeks after surgery, TSH was 2.05 with free T4 0.6, consistent with operative cure. Clinical Lessons: There is a wide differential diagnosis for non-suppressed TSH with elevated T4 and T3, including assay interference, familial syndromes including resistance to thyroid hormone beta (RTHβ), abnormal binding proteins, and TSHoma. Diagnostic testing includes repeating TFTs, obtaining a pituitary MRI, and using labs including SHBG and alpha subunit to differentiate between RTHβ and TSHoma. The first line treatment for TSHoma is TSS. In this patient with underlying Hashimoto’s disease and a TSHoma, TSH was somewhat suppressible with high-dose LT4 and the free T4 and total T3 were not always frankly high (appearing similar to subclinical hypothyroidism), delaying the time to diagnosis. References: 1. Amlashi FG, Tritos NA. Thyrotropin-secreting pituitary adenomas: epidemiology, diagnosis, and management. Endocrine. 2016; 52(3): 427-40. 2. Tjornstrand A, Nystrom HF. Diagnosis of Endocrine Disease: Diagnostic approach to TSH-producing pituitary adenoma. Eur J Endocrinol. 2017; 177(4): R183-R197.