Which laboratory test is primarily used to diagnose hyperthyroidism?

The primary laboratory test used to diagnose hyperthyroidism is the measurement of free thyroxine (FT4) levels. In hyperthyroidism, the thyroid gland is overactive and produces an excessive amount of thyroid hormones, particularly thyroxine (T4). Elevated levels of FT4 in the bloodstream indicate that the thyroid is functioning at an increased rate, which aligns with the diagnosis of hyperthyroidism.

Thyroid-stimulating hormone (TSH) is also important in evaluating thyroid function, as it regulates the production of thyroid hormones. In cases of primary hyperthyroidism, TSH levels are typically suppressed due to negative feedback from the high levels of circulating thyroid hormones. However, measuring FT4 directly provides a clearer assessment of the thyroid hormone levels that are elevated in hyperthyroidism.

Thyroid peroxidase antibodies are relevant in diagnosing autoimmune thyroid diseases, such as Hashimoto's thyroiditis, but they are not primarily used to diagnose hyperthyroidism. Serum calcium is unrelated to thyroid function and is more commonly assessed for parathyroid disorders or metabolic conditions. Therefore, the measurement of FT4 is the most direct approach for diagnosing hyperthyroidism.