Which hormone is increased in the diagnosis of hyperthyroidism?

In the context of diagnosing hyperthyroidism, an increase in Free T4 (thyroxine) is a key indicator. In hyperthyroidism, the thyroid gland is overactive, producing excessive amounts of thyroid hormones, including Free T4 and Free T3.

Free T4 is the unbound, active form of thyroxine that plays a significant role in regulating metabolism, and its elevation indicates that the thyroid is functioning at an increased capacity. While Free T3 also increases in hyperthyroidism, Free T4 is often measured as a marker due to its longer half-life and overall contribution to the physiological effects of thyroid hormone.

In contrast, TSH (thyroid-stimulating hormone) typically decreases in hyperthyroidism because of the negative feedback mechanism; when thyroid hormone levels rise, TSH production from the pituitary gland is suppressed. Thyroglobulin may be elevated in certain thyroid conditions but is not specifically used to diagnose hyperthyroidism. Hence, the significant increase in Free T4 directly correlates with the diagnosis of hyperthyroidism, making it the correct choice.