What is considered the most sensitive test for diagnosing hyperthyroidism?

The most sensitive test for diagnosing hyperthyroidism is the measurement of free T4. Free T4 refers to the fraction of thyroxine that is not bound to proteins in the blood, providing a more specific indicator of the active thyroid hormone available to tissues. This measurement is particularly important because it reflects the status of thyroid hormone production and metabolism more directly than total T4, which can be influenced by various factors, including protein levels.

In cases of hyperthyroidism, the body produces an excess of free T4, which leads to the clinical signs observed in affected patients. This makes free T4 a crucial test in confirming a diagnosis, especially when combined with clinical signs and other test results for thyroid function. Its high sensitivity allows for the detection of hyperthyroidism even in early or less obvious cases, signaling changes before total T4 levels may become altered.

While total T4 and nuclear scintigraphy provide useful information in evaluating thyroid function and gland activity, free T4's role in assessing active hormonal status makes it the preferred initial test for diagnosing hyperthyroidism. Thyroid stimulating hormone levels may also indicate hyperthyroidism, but they can sometimes be suppressed and do not provide the same direct assessment of thyroid hormone levels as free T4 does