What do the numbers mean?
| TSH level | Label | What it suggests |
|---|---|---|
| Below 0.4 mIU/L | Suppressed | Possible hyperthyroidism or over-treatment |
| 0.4–2.5 mIU/L | Optimal | Thyroid working well, most feel best here |
| 2.5–4.0 mIU/L | Normal | Within range, but some experience symptoms |
| 4.0–10 mIU/L + normal T4 | Subclinical hypothyroid | Early thyroid underfunction — often symptomatic |
| Above 10 mIU/L | Overt hypothyroid | Treatment almost always recommended |
TSH (thyroid-stimulating hormone) is your pituitary gland shouting at your thyroid to work harder. Elevated TSH with normal T4 means your thyroid is struggling to keep up — the pituitary is compensating. It's an early warning signal.
Normal vs Optimal — there is a difference
Most labs use a TSH range of 0.4–4.5 mIU/L. A result of 4.2 is "normal" by that standard. But many thyroid specialists and guidelines (including the British Thyroid Foundation) acknowledge most people feel well with TSH between 0.5 and 2.5 mIU/L.
Additionally, if only TSH is tested — and free T3 is not — you may be missing a significant part of the picture. T3 is the active hormone your cells actually use. Many patients with "normal" T4 have poor T4-to-T3 conversion.
Common symptoms you might notice
Common causes
- Hashimoto's thyroiditis — the most common cause, an autoimmune attack on the thyroid
- Iodine deficiency — less common in the UK but relevant globally
- Previous thyroid surgery or radioiodine treatment
- Certain medications (lithium, amiodarone)
- Post-pregnancy thyroiditis — often temporary
- Natural ageing — thyroid function declines gradually
Questions to ask your doctor
Copy these before your next appointment. Your doctor will appreciate that you came prepared.
- Can we also test free T3 and TPO antibodies to get a fuller picture?
- At what TSH level would you consider treatment — and why?
- Should we retest in 6–8 weeks, or is a trial of levothyroxine appropriate now?
- Could Hashimoto's explain my symptoms even if TSH is only mildly elevated?
- Is there anything I can do with diet or lifestyle while we monitor?
Have this marker in your results?
Get your full thyroid panel interpreted in plain English — TSH, T4, T3, and antibodies together. Upload your full report and get a plain-English breakdown of every value — plus a personalised question list for your doctor.