Thyroid Function Tests: Common Questions
Blood tests measuring TSH and thyroid hormones to assess thyroid activity.
Last revised
Thyroid Function Tests: the questions people actually ask, answered from published guidance.
Key takeaways
- TSH: rises when the thyroid is underactive, falls when overactive
- Subclinical hypothyroidism: raised TSH with normal T4
- Most useful single change: Ask for the actual numbers rather than "normal" or "abnormal".
- Commonest mistake: Interpreting TSH as if it moved in the same direction as thyroid hormone.
- See a doctor if any of the signs under "When to get help" below apply to you.
What is Thyroid Function Tests?
Blood tests measuring TSH and thyroid hormones to assess thyroid activity.
Why does it matter?
TSH moves in the opposite direction to the hormone it regulates, which is the single most confusing thing about the result.
Subclinical hypothyroidism for Thyroid Function Tests?
Raised TSH with normal T4.
Non-thyroidal illness for Thyroid Function Tests?
Can distort results; retest once recovered.
Biotin supplements for Thyroid Function Tests?
Can interfere with the assay.
Antibody testing for Thyroid Function Tests?
Helps identify autoimmune thyroid disease.
What is the single most useful thing to do?
Ask for the actual numbers rather than "normal" or "abnormal".
What is the most common mistake?
Interpreting TSH as if it moved in the same direction as thyroid hormone.
When should I see a doctor?
See a GP promptly if you are pregnant or planning pregnancy and have any thyroid abnormality, as targets differ and treatment often needs adjusting quickly. Seek urgent help for extreme drowsiness and confusion, or for fever with a very rapid heart rate and agitation.
Where to go next
- Thyroid Function Tests: Separating Myth From Guidance
- Starting Out With Thyroid Function Tests
- Thyroid Function Tests — the reference entry
- Thyroid Lumps: What Published Guidance Supports
- The Evidence on Polycystic Ovary Syndrome, Without the Hype
When to get help
See a GP promptly if you are pregnant or planning pregnancy and have any thyroid abnormality, as targets differ and treatment often needs adjusting quickly. Seek urgent help for extreme drowsiness and confusion, or for fever with a very rapid heart rate and agitation.
Where this comes from
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, summarised or generalised 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 a GP, 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 999.
