Challenges in the functional diagnosis of thyroid nodules before surgery for TSH-producing pituitary adenoma

SUMMARY: A 50-year-old woman with thyroid-stimulating hormone (TSH)-producing pituitary adenoma (TSHoma) was diagnosed due to symptoms of thyrotoxicosis. Preoperatively, she showed thyrotoxicosis with the syndrome of inappropriate secretion of TSH (SITSH) and had a 5 cm nodule in her thyroid gland. Octreotide was administered preoperatively, which helped lower her serum TSH level but not her thyroid hormone level. These findings were atypical for a patient with TSHoma. The TSHoma was completely resected, and the TSH level dropped below the sensitivity limit shortly after surgery. Interestingly, however, thyroid hormone levels remained high. A clear clue to the aetiology was provided by consecutive thyroid scintigraphy. Although preoperative thyroid scintigraphy did not show a hot nodule and the mass was thought to be a non-functional thyroid nodule, the nodule was found to be hot in the postoperative phase of TSH suppression. By focusing on the atypical postoperative course of the TSHoma, we were able to conclude that this was a case of TSHoma combined with an autonomously functioning thyroid nodule (AFTN). LEARNING POINTS: The diagnosis of autonomously functioning thyroid nodules (AFTNs) depends on suppressed serum TSH levels. If thyroid hormones are resistant to somatostatin analogue therapy or surgery for TSHoma, complications of AFTN as well as destructive thyroiditis need to be considered. It is important to revisit the basics when facing diagnostic difficulties and not to give up on understanding the pathology.