MON-125 A Rare Case of an Isolated Central Hypothyroidism from a Pituitary Macroadenoma on top of Papillary Thyroid Carcinoma

Abstract Disclosure: K.O. Tolentino: None. V.M. De Castro: None. Background: Central hypothyroidism (CH) from a pituitary macroadenoma is rare ranging from 1 in 80,000 to 120,000 individuals, however, there are no prior reports with this condition coexisting with a papillary thyroid carcinoma (PTCa). CLINICAL CASE: A 67 year woman had a 30 year history of a 1x1 cm neck mass that progressively enlarged later developing easy fatiguability. Initial tests showed normal TSH (0.56 uIU/mL. n 0.27-4.20) but low FT4 (0.77 ng/dL, n 0.93-1.71). Thyroid ultrasound showed a 9.5x 11.7 x 13.1cm right lobe with a 1.8 x 3.3cm ill-defined heterogenous solid nodule, and a 1.9 x 2.8cm ill-defined hypo- and hyperechoic nodule at the left. Neck CT showed a 9.3 x 11.4 x 12.9cm heterogenous right thyroid mass with calcifications and compression of adjacent structures. She was advised surgery but only sought consult after 6 months. Repeat testing showed 2 consecutively low FT4 (0.68 and 0.88 ng/dL), and TSH (0.038 and 0.021 uIU/mL, normal 8am cortisol (16.5 mcg/dL), ACTH (12.3 pg/mL), and PRL (12 mcg/L). Levothyroxine (LT4) 50mcg daily was given and a suppressed TSH <0.005 uIU/mL with normal FT4 1.014 ng/dL was observed after 1 month. A 9 x 12 x 9mm pituitary mass with unaffected optic chiasm was seen on MRI. Total thyroidectomy was done and histopathology revealed an 18.5 cm PTCa, follicular variant, infiltrative, at the right lobe with lymphovascular invasion (ATA high risk). LT4 100 mcg daily was initiated and a repeat CT 2 weeks later showed a lytic mass at the manubrium (4.8 x 4.5 x 4.8 cm) and right 1st anterior rib (3.1 x 4.2 x 3.2cm) with multiple subcentimeter solid, non-calcified lungs metastasis. An initial plan was to give recombinant TSH with RAI, although outcomes are mixed, with increased mortality in higher ATA risk (1), Moreover, in CH, there is decreased stimulation of the NIS, leading to poor iodine uptake on RAI scan and false negative results hence other imaging techniques (F-18 FDG PET/CT or 68Ga-PSMA-11) allows better findings (2,3). Due to the large remnant, referral to TCVS for resection was done but she was lost to follow up. Conclusion: CH remains to be a rare condition that can co-exist with a PTCa. De novo chromosomal mutation of oncogenes still remains to be the more likely cause. RAI can be considered in patients with low TSH, but outcomes are varied, with poorer outcomes in higher risk patients. Monitoring poses a challenge where the use of novel diagnostic imaging studies should be considered.REFERENCE: (1) Butt MI, et al. Papillary Thyroid Cancer With Pituitary Gland Metastasis: A Unique Encounter. Cureus. 2023 Apr 27;15(4)(2) Iqbal A, Rehman A. Thyroid Uptake and Scan. StatPearls. Treasure Island (FL): StatPearls Publishing; 2025 Jan(3) Pitalua-Cortes Q, et al. Head-to-Head Comparison of 68Ga-PSMA-11 and 131I in the Follow-Up of Well-Differentiated Metastatic Thyroid Cancer: A New Potential Theragnostic Agent. Front Endocrinol (Lausanne). 2021;12:794759. Published 2021 Dec 22 Presentation: Monday, July 14, 2025