Which test is considered the most sensitive for diagnosing hyperthyroidism?

The most sensitive test for diagnosing hyperthyroidism is Thyroid Stimulating Hormone (TSH). In cases of hyperthyroidism, the levels of TSH are typically suppressed due to the negative feedback mechanism exerted by elevated thyroid hormone levels (T3 and T4) on the pituitary gland. Thus, a low level of TSH indicates that the thyroid is producing too much hormone, which is a hallmark sign of hyperthyroidism.

While T3 and T4 levels also play a crucial role in the diagnosis, they are less sensitive indicators compared to TSH because they can vary in early hyperthyroidism or in subclinical cases where the TSH may drop before significant changes in the levels of T3 or T4 are detected. Thyroid antibodies can provide valuable information about autoimmune thyroid conditions, such as Graves' disease, but they are not utilized specifically for routine hyperthyroidism diagnosis.

This makes TSH the most effective and sensitive initial screening test for hyperthyroidism, allowing for appropriate diagnosis and management.