A patient has an elevated serum T3 and free T4 and undetectable TSH. What is the most likely cause of these results?

In the given scenario, the patient presents with elevated serum T3 (triiodothyronine) and free T4 (thyroxine) levels combined with undetectable TSH (thyroid-stimulating hormone). This specific pattern is indicative of primary hyperthyroidism.

In primary hyperthyroidism, the thyroid gland is overactive and produces excess amounts of thyroid hormones (T3 and T4), which leads to negative feedback on the pituitary gland, resulting in suppressed levels of TSH. This negative feedback mechanism is central to understanding the diagnosis.

The elevated levels of T3 and free T4 reflect an autonomous production of these hormones by the thyroid itself, which is characteristic of conditions such as Graves' disease or toxic adenomas. The undetectable TSH confirms that the feedback loop is effectively inhibiting its secretion due to high levels of circulating thyroid hormones.

Other conditions such as secondary hyperthyroidism could present with elevated T3 and T4, but they would usually have elevated TSH as well due to a pituitary cause of hyperthyroidism. In contrast, conditions like increased thyroxine-binding proteins or euthyroid sick syndrome would not typically exhibit such high thyroid hormone levels or the specific suppression of TSH