Why is T3 level more reliable in testing hyperthyroidism?

T3 levels are not more reliable in testing for hyperthyroidism. Free thyroxine (FT4) and thyroid-stimulating hormone (TSH) are the preferred tests for evaluating thyroid function. T4 is the main thyroid hormone responsible for the body's metabolic rate, and FT4 measures the amount of free, unbound T4 available in the blood. TSH is produced by the pituitary gland in response to changes in thyroid hormone levels, and high levels of TSH can indicate an underactive thyroid gland, while low levels can suggest an overactive thyroid gland. Therefore, FT4 and TSH are typically the most accurate and reliable tests for diagnosing thyroid conditions, including hyperthyroidism.