Differential Diagnosis:
In this case, differential diagnoses include primary hypothyroidism with pituitary hyperplasia, as chronic elevated TSH from untreated hypothyroidism can cause pituitary enlargement. A TSH-secreting pituitary adenoma (TSHoma) is less likely, as thyroxine therapy normalized TSH, which wouldn’t occur with a true TSHoma. The stable, non-enhancing lesion may represent a non-functional pituitary microadenoma coexisting with hypothyroidism, while secondary hypothyroidism due to compression by the lesion is improbable given the lesion’s small size and lack of compression. Granulomatous diseases, like sarcoidosis, can involve the pituitary, causing headaches and endocrine dysfunction, though they often present with more extensive hormonal deficiencies than isolated TSH elevation.