Why TSH alone isn't enough
TSH comes from the pituitary and tells the thyroid how hard to work. The gland mostly makes T4, a storage form, which the liver, gut and tissues convert to T3, the active hormone every cell responds to. TSH shows the request, free T4 shows the output and free T3 shows what the cells receive. A TSH inside the range can sit alongside a free T3 that is drifting down, which is why free T4 and free T3 are worth adding when TSH is moving or symptoms don't fit. The Vively Baseline includes TSH, and the Thyroid Function Test adds free T4 and free T3.
What each marker tells you
| Marker | What it shows | Typical lab range | Vively optimal |
|---|---|---|---|
| TSH | The pituitary's signal to the thyroid | 0.5 to 4.0 mIU/L | 1.0 to 2.5 mIU/L |
| Free T4 | Hormone output of the gland | 10 to 25 pmol/L | 12 to 22 pmol/L |
| Free T3 | Active hormone available to cells | 3.1 to 6.8 pmol/L | 4.0 to 5.5 pmol/L |
| Reverse T3 | An inactive form that rises with illness, under-eating and stress | 230 to 540 pmol/L | 320 to 450 pmol/L |
| TPO and thyroglobulin antibodies | Immune activity against the gland | Negative, or below the laboratory's cut-off | No separate optimal range |
Ranges vary by laboratory and method, so read results against the range printed on the client's report. Vively optimal ranges are a second comparison for follow-up, not diagnostic cut-offs, and the TSH optimal range doesn't apply in pregnancy or to adults over 70.
Four patterns and what to do next
| Pattern | What it suggests | Next step |
|---|---|---|
| TSH high, free T4 low | An underactive thyroid (primary hypothyroidism) | Back to the GP |
| TSH normal or drifting up, antibodies positive | Early autoimmune thyroiditis (Hashimoto's), often years before TSH crosses the line | Make sure the GP knows, and monitor. Check iron, selenium, iodine, progesterone and FSH |
| TSH and free T4 normal, free T3 low | T4 is not converting well to T3 | Address under-eating, illness, stress, low iron or low selenium. Check a Day 21 progesterone, then retest |
| TSH low | An overactive thyroid, or too much thyroid replacement | Back to the GP |
Always read TSH with free T4, free T3, symptoms, age and the trend across tests. Low levels of antibodies don't always mean disease, but they do mean the immune system is involved and the result deserves watching.
Poor conversion and the co-nutrients
When TSH and free T4 are normal but free T3 is low, T4 may not be converting well to T3. Common drivers are under-eating (including with GLP-1 medicines), a recent illness, chronic stress and low iron or selenium. A high reverse T3 next to a low free T3 supports a conversion problem, though reverse T3 is optional and its value is debated in mainstream practice. Read it only next to free T3.
- Iron: thyroid peroxidase, the enzyme that builds thyroid hormone, needs iron. Low ferritin slows the thyroid down.
- Selenium: powers the enzymes that convert T4 to T3 and helps protect the gland. See selenium.
- Iodine: the raw material. Too little and too much both cause problems, so don't supplement it blind.
- Zinc: supports hormone production and conversion. See zinc.
Fix low ferritin before looking for a thyroid answer to fatigue, and find out why it is low. The iron studies guide covers how to read it.
Getting an accurate result
- On thyroxine: don't stop it. Draw blood in the morning before that day's dose, then take it. Free T4 rises for a few hours after a dose.
- Biotin: common in hair, skin and nail formulas and many B complexes, it can falsely raise free T4 and lower TSH. Pause it for 3 to 5 days before the test.
- Same laboratory, morning draw: TSH runs higher earlier in the day, so keep the laboratory and the time of draw consistent when you are tracking a trend.
- Wait 6 to 8 weeks after a change: a new dose, a new supplement or an acute illness. TSH takes that long to settle.
Thyroid across life stages
From the mid-thirties, progesterone tends to fall years before oestrogen becomes erratic, and thyroid changes often show up in the same years. With an early Hashimoto's or poor conversion pattern in a woman of that age, it is worth also checking a well-timed progesterone and FSH. See the hormone test timing guide for when to draw them.
Worked example: Sarah
Sarah is an example client, not a real person.
Sarah is 44, tired, foggy and gaining weight around the middle. Over three years her TSH went 2.4, 2.9 and 3.4 mIU/L, and on her latest test it was 3.5. Her free T4 fell from 15 to 13 pmol/L and her free T3 from 4.6 to 4.0 pmol/L. Every value is inside the laboratory range. TSH is above Vively's optimal range, free T4 sits low in it and free T3 is right at its lower edge.
Her ferritin is 22 µg/L and falling, so her iron comes first. The sharper question is whether this is autoimmune, so the next step is TPO and thyroglobulin antibodies, added in the platform or requested by her GP. If they are positive, she is monitored more closely and her iron and selenium matter even more. Thyroid is retested in 6 months, alongside her lifestyle changes.
When to refer back to the GP
- TSH above 5 or below 1 mIU/L.
- Free T4 outside the laboratory range.
- High TSH with low free T4, or a low TSH: both need medical assessment.
- Positive thyroid antibodies, so the GP knows and can monitor.
Follow the reporting laboratory and your own scope of practice.
Source: Vively Practitioner Masterclass Part 2 with Dr Michelle Woolhouse, recorded 21 October 2026, and its cheat sheet (PDF). See the full session.
Request practitioner access to refer these markers for your own clients. You need to be a verified practitioner to refer the tests.
Is a TSH of 3.5 mIU/L normal?
It is inside the typical laboratory range of 0.5 to 4.0 mIU/L, and above Vively's optimal range of 1.0 to 2.5 mIU/L. Read it with free T4, free T3, antibodies, symptoms and the trend across tests rather than on its own.
Should I request reverse T3?
It is optional. Reverse T3 rises with illness, under-eating and stress, and its value is debated in mainstream practice. If you use it, read it only next to free T3.
How long should a client stop biotin before a thyroid test?
At least 3 to 5 days. Biotin is in many hair, skin and nail formulas and B complexes, and it can falsely raise free T4 and lower TSH.
Should a client skip their thyroxine before a blood test?
No. They should have the blood drawn in the morning before that day's dose, then take it.
When should I retest after a change?
Wait 6 to 8 weeks after a new dose, a new supplement or an acute illness, because TSH takes that long to settle.
- British Thyroid Foundation. Thyroid antibodies explained.
- American Thyroid Association. Biotin and thyroid function tests.
- Royal College of Pathologists of Australasia. Manual of use and interpretation of pathology tests: thyroid function.