What laboratory test should be ordered first when diagnosing thyroid conditions?

In the evaluation of thyroid function, the measurement of Thyroid Stimulating Hormone (TSH) is typically the most critical first step. TSH is produced by the pituitary gland and regulates the function of the thyroid gland. When a healthcare provider suspects a thyroid condition, measuring TSH levels provides insight into how the thyroid is responding to signals from the body.

An elevated TSH level usually indicates an underactive thyroid (hypothyroidism), while a low TSH level may suggest an overactive thyroid (hyperthyroidism). By beginning with TSH testing, clinicians can determine if there is a disorder at the level of the thyroid or the pituitary.

If TSH results indicate an abnormality, further testing, such as free T4 and T3 levels, along with thyroid autoantibody tests like TPO, can help refine the diagnosis. However, initiating the workup with TSH is essential as it guides all subsequent testing and is a reliable marker of thyroid health.