Differential Diagnosis:
The differential diagnoses for this case include a TSH-secreting pituitary adenoma (TSHoma), which is supported by the elevated FT4 and FT3 levels, persistent hyperthyroid symptoms, and the sellar mass on MRI, making it the most likely diagnosis. Resistance to thyroid hormone (RTH) is another consideration, as it can cause elevated thyroid hormones with unsuppressed or high TSH, but typically lacks the clinical symptoms of hyperthyroidism, such as tachycardia and tremors, and does not show a pituitary mass on imaging. Exogenous thyroid hormone ingestion (factitious thyrotoxicosis) could also lead to high FT4 and FT3, but would typically suppress TSH and is unlikely to be associated with a pituitary mass. Lastly, multinodular goiter with autonomous thyroid function could cause elevated thyroid hormones with a mildly elevated TSH, but this diagnosis is improbable due to the absence of multinodular findings