SAT-481 TSH Adenoma: An Elusive Diagnosis

Background: Thyroid stimulating hormone (TSH)-secreting tumors make up less than 2% of pituitary tumors. Reported cases of these tumors in patients with Grave’s disease are exceedingly rare. This case highlights the importance of considering the diagnosis in these patients. Case: A 66 year-old Caucasian female with history of Grave’s disease presented to our endocrine clinic in November 2013 for evaluation of post-ablative hypothyroidism, treated with levothyroxine (LT4). Despite higher doses of LT4, her TSH remained elevated, as high as 15.3 mIU/L (0.358-3.74 mIU/L.) She had weight gain and fatigue. Her free thyroxine (FT4) was elevated, as high as 2.13 ng/dL (0.76-1.46 ng/dL.) She denied palpitations, diarrhea, tremors, headaches, or vision changes. FT4 by equilibrium dialysis was confirmed elevated at 3.7 ng/dL. TSH assay interference was ruled out by negative heterophile antibodies. As labs showed persistent TSH and FT4 elevation without correlation to LT4 dose, other diagnoses were considered. Alpha-subunit, prolactin, cortisol, insulin-like growth factor-1 were all normal. Thyroid hormone resistance mutation was negative. A 5 mm pituitary lesion was noted on Magnetic Resonance Imaging (MRI) in 2010; she refused imaging follow up. Visual field testing showed no deficits. Suspicion rose for a TSH-secreting pituitary adenoma (TSH-oma.) LT4 was titrated to normalize FT4 and prevent iatrogenic hyperthyroidism. She agreed to an MRI in 2016, that showed interval growth of the pituitary mass to 1 cm. TSH and FT4 in October 2016 were 52 mIU/L and 1.80 ng/dL, respectively. Repeat alpha-subunit was high. Neurosurgery referral was made. Surgery was deferred due to lack of compressive symptoms and need for lifelong anticoagulation due to multiple thromboses. She began Lanreotide to reduce TSH and shrink the tumor. Repeat MRI showed decreased adenoma size after completion of Lanreotide. TSH and FT4 improved to 13.60 mIU/L and 1.01 ng/dL, respectively. Discussion: This case demonstrates the diagnostic difficulty that arose when clinical and laboratory features did not correlate. The patient’s lack of symptoms of hyperthyroidism are in contrast to the typical presentation of a TSH-oma. This was complicated by initially normal alpha-subunit and pituitary hormones, refusal by patient for follow up pituitary imaging, and lack of compressive symptoms. TSH-omas are rare, and coexistence in patients with Grave’s disease is more rare. Thus, it is vital to consider this entity to avoid clinical complications of missing the diagnosis. Reference: Camacho, P. M., Gharib, H., & Sizemore, G. W. Evidence-based endocrinology. 2012; 3; 11-12.