THU096 When TSH Alone Is Not Enough

Disclosure: S.S. Awasty: None. L. Ghalib: None. Background: A Thyrotropinoma or TSHoma is the rarest of all pituitary adenomas approximately 1-2%. TSHomas are often only found due to symptoms from co-secreting tumors with TSHomas or routine laboratory testing. Misinterpretation of laboratory values can cause misdiagnosis and inappropriate treatment. Clinical Case: A 45-year-old female presented to her primary care physician for a routine appointment. The patient reported concerns regarding weight and a TSH was ordered, TSH 7.00 uIU/mL (0.49 - 4.67) with no other thyroid function values. The physician prescribed levothyroxine 50mcg treating for primary hypothyroidism and over the next 37 months the patient was prescribed increasing doses of levothyroxine based on TSH values. This patient was eventually referred to Endocrinology. At the initial Endocrinology visit TSH 7.59 uIU/mL (0.27 - 4.2 uIU/mL) and fT4 3.2 ng/dL (0.7 - 1.7) on levothyroxine 125mcg. Levothyroxine was discontinued and labs were repeated 6 weeks later TSH 5.9 uIU/mL (0.27 - 4.2), fT4 4.8 ng/dL (0.7 - 1.7), fT3 2.1 pg/mL (2 - 4.4), and alpha-subunit 1.5ng/mL (<=1.2 premenopausal female, <=1.8 postmenopausal female). A pituitary macroadenoma was discovered on MRI. The patient was referred to Pituitary Clinic and noted to have a prolactin of 53.8 ng/mL (non-pregnant females 2.8-29.2) and IGF-1 291.6 ng/mL (60.0 - 240.0). The patient underwent transsphenoidal pituitary surgery; pathology reported a plurihormonal subtype pituitary adenoma which stained positive for Prolactin, HGH, and TSH. One week after removal of the TSHoma, TSH 0.014 uIU/mL (0.550 - 4.780) fT4 0.65 ng/dL (0.89 - 1.76), and levothyroxine replacement was initiated with plans for continued outpatient monitoring to assess need for levothyroxine replacement. Conclusion: This case primarily demonstrates the importance of evaluating TSH levels in conjunction with free T4 levels to effectively interpret central versus primary causes of thyroid dysfunction. The high molar ratio of the alpha-subunit to the TSH level lends itself to a TSHoma differentiating it from a diagnosis of syndrome of resistance to thyroid hormone. While rare missing a TSHoma diagnosis could have devastating consequences. Checking other pituitary hormones pre-operatively even without symptoms also gave the indication that this was a co-secreting tumor. After adenoma removal rechecking pituitary hormones is of great importance as exemplified by pituitary-thyroid axis suppression noted post-operatively in this case. Presentation: Thursday, June 15, 2023