Understanding Thyroid Function Tests
Thyroid-stimulating hormone, or TSH, is often the first blood test ordered. Tests for thyroid hormones such as free T4 or T3 may help clarify the result. Thyroid antibody tests investigate possible causes and are not the same as measuring hormone function.
How it is assessed or used in care
Testing can help evaluate an underactive or overactive thyroid and monitor treatment. Results depend on context, including illness, pregnancy, medications, and supplements. Not every symptom of fatigue or weight change is caused by the thyroid.
What to discuss with your clinician
Tell your clinician about medications and supplements, including biotin, which can interfere with some tests. Ask whether any preparation is needed and how results affect your treatment. Do not change thyroid medication without a clinician’s guidance.
Medical references
Sources accessed November 13, 2026.
This information is for general education, not a diagnosis or an individualized treatment plan. Discuss symptoms, test results, and treatment decisions with a qualified clinician. For a medical emergency, call 911.