The Short Guide to Thyroid Function Tests
Thyroid Function Tests: the whole thing in five minutes, for when you want the numbers and nothing else.
Blood tests measuring TSH and thyroid hormones to assess thyroid activity.
Key takeaways
- TSH: rises when the thyroid is underactive, falls when overactive
- Subclinical hypothyroidism: high TSH with normal T4
- Most useful single change: Ask for the actual numbers rather than "normal" or "abnormal".
- Commonest mistake: Reading TSH as if it moved in the same direction as thyroid hormone.
- See a doctor if any of the signs under "When to speak to a doctor" below apply to you.
The numbers, in one place
| Measure | Figure |
|---|---|
| TSH | rises when the thyroid is underactive, falls when overactive |
| Subclinical hypothyroidism | high TSH with normal T4 |
| Acute illness | can distort results; retest once recovered |
| Biotin supplements | can interfere with some lab tests, including thyroid tests |
| Antibody testing | helps identify autoimmune thyroid disease |
In one paragraph
TSH moves in the opposite direction to the hormone it regulates, which is the single most confusing thing about the result.
Worth doing
- Ask for the actual numbers rather than "normal" or "abnormal"
- Tell your doctor if you take biotin, and ask whether to stop it before testing
- Retest after an acute illness rather than acting on a result taken during it
- Ask whether antibodies were tested if the picture is borderline
Worth avoiding
- Reading TSH as if it moved in the same direction as thyroid hormone
- Acting on a single borderline result
- Buying direct-to-consumer thyroid panels and self-treating
Related topics
- Testosterone: What the Evidence Actually Says (Men's Health)
- Blood Tests Explained: Sorting the Evidence From the Noise (Blood Health)
- What the Guidance Really Says About Understanding eGFR (Kidney Health)
- Having a Blood Test: Sorting the Evidence From the Noise (Using Health Services)
Where to go next
- Thyroid Function Tests: Separating Myth From Guidance
- Starting Out With Thyroid Function Tests
- Thyroid Function Tests — the reference entry
- Thyroid and Hormones: The Complete Guide
- Thyroid Lumps: What Published Guidance Supports
- The Evidence on Polycystic Ovary Syndrome, Without the Hype
When to speak to a doctor
See your doctor promptly if you are pregnant or planning pregnancy and have any thyroid abnormality, as targets differ and treatment often needs adjusting quickly. Call 911 for extreme drowsiness and confusion, or for fever with a very rapid heart rate and agitation.
Where this comes from
- American Thyroid Association — Thyroid Function Tests
- FDA — The FDA Warns that Biotin May Interfere with Lab Tests
How this page was put together
Every figure, recommendation and warning above is taken from the guidance named in Where this comes from. Nothing on this page is inferred, summarized or generalized beyond what those bodies publish: where they give a number, it is their number, and where they are silent, so are we. Pages are rebuilt whenever the underlying record changes. Sign-off by a named clinician, where it has happened, is shown at the top of the page along with the date.
Medical disclaimer
This article is general information, not medical advice. It cannot account for your own history, medication or circumstances. Always speak to your doctor, pharmacist or another qualified health professional before making changes that affect your treatment, and never delay seeking medical advice because of something you have read here. In an emergency, call 911.
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