Advanced Magnetic Resonance Imaging Techniques for Localization of a Small Thyrotropin-Secreting Pituitary Tumor

Thyrotropin (TSH)-secreting pituitary tumors (TSH-omas) are a rare cause of hyperthyroidism. Historically, the majority have been macroadenomas (>1 cm); however, microadenomas (<1 cm) are increasing in frequency. Localization of TSH-secreting microadenomas can be challenging. We report the first case employing advanced pituitary magnetic resonance imaging (MRI) sequences to localize a TSH-secreting microadenoma, leading to successful resection. The patient presented with mild symptoms of hyperthyroidism and biochemistry confirming secondary hyperthyroidism. α-Glycoprotein subunit and sex hormone-binding globulin were elevated; however, cross-linked C-telopeptide of type I collagen was within the reference range. No pathogenic variants in thyroid hormone resistance genes were detected. A triiodothyronine suppression test, thyroid hormone-releasing hormone stimulation test, and short-acting octreotide suppression test were consistent with a TSH-oma. Initial pituitary MRI did not demonstrate a pituitary tumor. However, advanced imaging techniques including coronal T1 3-dimensional volumetric interpolated breath-hold sequence (VIBE) thin-slice scans confirmed a small, right-sided pituitary tumor. Transsphenoidal surgery was performed, and histopathology confirmed a TSH-oma. The patient remains in biochemical remission. TSH-secreting microadenomas are increasing in frequency and localization is likely to become an increasing challenge. Advanced MRI imaging techniques appear beneficial in the localization of TSH-secreting microadenomas.