MON-058 TSH/GH Co-Secreting Pituitary Adenoma: Challenges in Medical Management of Residual Disease

Abstract Disclosure: J. Prim: None. T.D. Hoang: None. Background: Co-secretion of thyroid stimulating hormone (TSH) and growth hormone (GH) from a pituitary adenoma [(GH+TSH)oma] is rare. These tumors tend to have larger size and more aggressive invasive features at time of diagnosis, making surgical cure difficult. In the case of a residual (GH+TSH)oma after surgery and radiotherapy, the development of partial remission of GH hyperfunction with the simultaneous development of hypothyroidism on somatostatin analog therapy is an uncommon clinical scenario creating therapeutic challenges not well-described in the existing literature. Case Presentation: A 63-year-old man presented for management of residual TSH and GH co-secreting pituitary macroadenoma. The tumor was discovered during diagnostic evaluation of symptoms of progressive fatigue and anxiety. Pituitary function testing revealed FT4 2.0ng/dL, TSH 8.3 mIU/mL, total testosterone 113ng/dL, IGF-1 435 ng/dL, and alpha subunit 1.4 ng/mL. MRI revealed a 3.5 x 2.9 x 3.4 cm sellar mass with invasion into the base of sphenoid and suprasellar extension with flattening of the optic chiasm. He was started on somatostatin analog therapy and underwent transsphenoidal resection with debulking of 80% of the tumor. This was followed by radiation therapy, after which somatostatin analog therapy was stopped. 18 months after surgery, laboratory testing showed TSH 6.1 mIU/mL, FT4 1.81ng/dL, and normal IGF-1 121 ng/dL, and lanreotide was restarted. After several years of biochemical euthyroidism, his FT4 level declined to a range of 0.5-0.8ng/dL with TSH 1.95-3.91 mIU/mL and IGF-1 118ng/dL. Repeat MRI eight years after surgery showed stable posttreatment changes and a 5mm residual adenoma. The patient was reluctant to pursue any further invasive therapies or add new medications to his regimen, so he has been managed with adjustments in the interval between lanreotide injections and assessment of response. Discussion: Complete remission occurs in only one third of patients after surgical resection of (GH+TSH)oma, with most patients continuing to display biochemical hormonal hyperfunction of residual tumor. Somatostatin analogs have been shown to effectively reduce serum TSH, thyroid hormone concentrations, and IGF-1 levels in these cases. The interval development of hypothyroidism with IGF-1 levels remaining in the normal range creates a clinical challenge, since dose or frequency reduction in the somatostatin analogue may allow for normalization of thyroid function but may also result in GH and IGF-1 elevation. A potential role for levothyroxine replacement has been described for management of biochemical hypothyroidism from somatostatin analogs but data is lacking for its use in the context of a (GH+TSH)oma. This case highlights the difficulties in management of these rare hormone co-secreting pituitary adenomas and the need for multi-disciplinary care and close endocrinology follow up. Presentation: Monday, July 14, 2025