Thyroid Guides
TSH, T4, T3, thyroid antibodies, and thyroid conditions.
What these guides cover
Thyroid guides cover TSH, free T4, free T3, and thyroid antibodies. Thyroid testing is common for fatigue, weight changes, and temperature sensitivity. The guides explain how the tests fit together and why one marker alone rarely tells the full story.
Thyroid testing usually begins with TSH, a hormone made by the pituitary gland that responds to thyroid hormone levels. If TSH is outside the usual range, a clinician may add free T4 or free T3 to see how the thyroid itself is responding. Antibody tests, such as anti-TPO, look for immune activity that can affect the thyroid over time. No fasting is generally needed, though some medications can influence results, so tell the collection staff what you take.
How to use these guides
Each guide below focuses on a single test or a small group of related tests. Start with the marker you and your clinician have discussed, then follow the related-test links to see what is often ordered alongside it. Preparation and timing instructions appear in each guide, and where testing is available is listed by state and city. These guides are educational and do not replace advice from a healthcare provider.
Hypothyroidism Blood Tests: How Thyroid Is Checked
Learn how hypothyroidism blood tests work, which thyroid markers are checked, and what results can mean before you order thyroid testing at
TSH Test Guide: What Thyroid Test Results Really Mean
Understand the TSH test, why it is the first-line thyroid screen, how it relates to T3 and T4, and how to order thyroid testing at a lab nea
Thyroid Antibody Testing: TPO and Thyroglobulin Tests
Learn how thyroid antibody testing works, what TPO and thyroglobulin antibodies suggest, and how to order a thyroid antibody panel at a lab
Thyroid FAQ
Why is TSH usually the first thyroid test?
TSH is the pituitary signal that rises or falls in response to thyroid hormone levels. Because it is sensitive to small changes, it is often used as a first screening marker before more specific tests are added.
Why would more than one thyroid marker be ordered?
TSH shows the pituitary response, while free T4 and free T3 show the hormones the thyroid produces. Reading them together helps a clinician see where in the feedback loop a change may be occurring.
How do I prepare for a thyroid test?
Thyroid tests generally need no fasting. If you take thyroid medication or supplements containing biotin, tell the collection staff, since timing and certain supplements can affect the measured values.
Reviewed by the LabDirectories Editorial Team
Our team researches and fact-checks all content to ensure accuracy. We update guides regularly to reflect current testing options, pricing, and laboratory accreditation standards.