FRI476 Asymptomatic Thyrotropin-secreting Pituitary Microadenoma With Biochemical Secondary Hyperthyroidism: A Diagnostic Conundrum

Disclosure: G. Al-Naqeeb: None. R. Ghosh: None. P. Veeraraghavan: None. C. Cochran: None. J. Klubo-Gwiezdzinska: None. S. Gubbi: None. Background: Thyrotropin (TSH)-secreting pituitary adenomas (TSHAs) are often macroadenomas (>1 cm), and clinically present with secondary hyperthyroidism (SH), but clinically silent microTSHAs (<1 cm) with only biochemical SH pose a diagnostic challenge. Clinical Case: A 49-year-old, healthy male was referred to our center for evaluation of abnormal thyroid function tests (TFTs). Immunoassay (IA) measurements at an outside hospital had identified elevated free T4 (FT4), and an inappropriately normal TSH, and a pituitary magnetic resonance imaging (MRI) had noted an 8mm x 7mm non-invasive adenoma. The patient denied hyperthyroidism symptoms, medication/herbal supplement intake, radiation exposure, headaches, vision changes, and thyroid/pituitary disorders, or abnormal TFTs in relatives. Physical exam revealed no signs and symptoms of hyperthyroidism. No thyromegaly or cervical lymphadenopathy was felt. IA profile at our institute was consistent with an SH pattern [TSH: 3.32 mIU/L (NL: 0.4 - 4.7); FT4: 3 ng/dL (NL: 0.9 - 1.7); total T4: 14.5 mcg/dL (NL: 4.5 - 11.7); triiodothyronine: 266.6 ng/dL (NL: 80 - 200)], with similar values on repeat measurement. Equilibrium dialysis technique also yielded an elevated FT4 value. TSH-receptor, anti-thyroid peroxidase, and anti-thyroglobulin (Tg) antibody (Ab) titers, serum Tg, and thyroid binding globulin levels were normal. Dilution studies ruled out heterophile Ab interference. Familial dysalbuminemic hyperthyroxinemia was unlikely given the elevated FT4 values. Genetic testing revealed a normal THRB gene, thereby ruling out thyroid hormone resistance. THRA gene was not tested due to absence of behavioral or hearing issues. Markers of hyperthyroidism, including angiotensin converting enzyme, carotene, apolipoproteins, and osteocalcin levels were normal, except for an elevated sex hormone binding globulin of 104 nmol/L (NL: 11 - 78). Biochemical evaluation of other pituitary axes was normal. However, an elevated alpha sub-unit fraction (ASF) was noted on two separate measurements. Sonogram revealed a heterogeneous, hypervascular thyroid. Bone densitometry, echocardiogram, and Holter monitor studies were normal. The combination of biochemical SH, elevated ASF, pituitary adenoma, and exclusion of other etiologies causing SH pattern established the diagnosis of TSHA. An active surveillance approach was implemented. A pituitary MRI done 18 months later showed minimal (2mm in one axis) increase in the adenoma size, and the patient continued to demonstrate biochemical SH but remained asymptomatic. Conclusion: MicroTSHAs can present with asymptomatic SH. In this situation, excluding other biochemical SH etiologies is crucial to prevent unnecessary invasive procedures. But once TSHA diagnosis is established, due to the risk of tumor growth leading to symptomatic SH or mass effects, active surveillance is warranted. Presentation: Friday, June 16, 2023