Starting Out With Thyroid Function Tests
Thyroid Function Tests: the few things that matter most, and the mistakes that catch people out early.
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: 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 speak to a doctor" below apply to you.
Why bother
TSH moves in the opposite direction to the hormone it regulates, which is the single most confusing thing about the result.
Start here
Two to start. There is no prize for doing all of it in week one.
- Ask for the actual numbers rather than "normal" or "abnormal"
- Stop high-dose biotin supplements before testing
Then add these
Next, once the first two have stopped feeling like effort.
- Retest after acute illness rather than acting on a result taken during it
- Ask whether antibodies were tested if the picture is borderline
The numbers, in one place
| Measure | Figure |
|---|---|
| TSH | rises when the thyroid is underactive, falls when overactive |
| Subclinical hypothyroidism | raised TSH with normal T4 |
| Non-thyroidal illness | can distort results; retest once recovered |
| Biotin supplements | can interfere with the assay |
| Antibody testing | helps identify autoimmune thyroid disease |
Common early mistakes
- Interpreting TSH as if it moved in the same direction as thyroid hormone
- Acting on a single borderline result
- Buying private thyroid panels and self-treating
Where to go next
- Thyroid Function Tests: Separating Myth From Guidance
- A Q and A on 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 speak to 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 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.
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