SAT-104 - When TSH Elevation Defies Explanation: Insights from A Patient with A TSH-Oma

Summary:
Papillary thyroid carcinoma (PTC) is the most common thyroid cancer and usually carries an excellent prognosis, though recurrence occurs in 5–21% of cases. Standard post-treatment care includes TSH suppression with levothyroxine, particularly in high-risk patients, where TSH targets are < 0.1 μIU/mL. Rarely, thyrotropin-secreting pituitary adenomas (TSH-omas) can cause elevated or inappropriately normal TSH despite elevated thyroid hormone levels, creating diagnostic confusion in patients with a history of thyroid cancer.

Case Presentation:
We describe a 56-year-old male with prior PTC and pituitary macroadenoma who developed persistently elevated TSH despite biochemical evidence of hyperthyroxinemia. He underwent total thyroidectomy with radioactive iodine ablation in 2007 and central neck dissection for recurrence in 2011. Follow-up was initially unremarkable.

In 2015, while on 224 mcg/day of levothyroxine, he had a suppressed TSH of 0.34 μIU/mL. By 2019, TSH rose to 5.12 μIU/mL despite an elevated FT4, leading to suspicion of medication nonadherence. No dosing changes were made.

In April 2020, following visual changes, MRI revealed a 2.2 cm pituitary macroadenoma compressing the optic chiasm. Transsphenoidal resection showed CAM5.2 positivity with mild prolactin staining; TSH staining was unavailable, and the tumor was labeled nonfunctioning. Postoperative hormone function was largely preserved.

By 2024, MRI showed possible tumor regrowth, and TSH levels continued to rise despite elevated FT4. Visual symptoms returned in 2025, accompanied by a second episode of atrial fibrillation. Labs showed persistent elevation of TSH (6.49 μIU/mL) with elevated FT4. Due to missing initial TSH immunostaining, alpha-subunit testing was performed and markedly elevated, supporting a diagnosis of TSH-oma.

Repeat MRI in 2025 confirmed residual invasive tumor involving the left sella and cavernous sinus with mass effect on the optic apparatus.

Thyroid Labs Over Time:
2015: TSH 0.34 / FT4 1.43
2019: TSH 5.12 / FT4 2.0
2024–2025: TSH 5.5–6.8 with elevated FT4 (1.7–2.2)

Discussion:
This case illustrates a rare overlap of TSH-oma and prior differentiated thyroid cancer. Diagnosis was delayed due to its rarity, absent TSH staining, and initial assumptions of noncompliance. Ultimately, rising TSH with elevated FT4, elevated alpha-subunit, and imaging confirmed a TSH-secreting adenoma. This case underscores the need for individualized follow-up in thyroid cancer survivors and awareness of TSH-omas as an uncommon but important cause of persistent TSH elevation after thyroidectomy.

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