Thyroid Report: Normal TSH Range and How to Read FT4
Learn the normal TSH range, how FT4 changes interpretation, pregnancy considerations and thyroid test preparation.
A thyroid report can be confusing when TSH is marked high or low. TSH should not be read alone; FT4, the laboratory reference range, symptoms, pregnancy status and previous results all matter.
MedlinePlus lists a common adult TSH reference range of about 0.4–4.8 µIU/mL, but laboratories vary. Start with the range printed on your report.
TSH, FT4 and FT3
| Test | What it reflects |
|---|---|
| TSH | The pituitary signal to the thyroid |
| FT4 | Available thyroxine in blood |
| FT3 | The active thyroid hormone |
Exeter Clinical Laboratory describes TSH as a first-line screening test; FT4 helps interpret an abnormal TSH.
How TSH and FT4 fit together
| TSH | FT4 / FT3 | Common pattern |
|---|---|---|
| High | FT4 low | Primary hypothyroidism pattern |
| High | FT4 normal | Subclinical hypothyroidism pattern |
| Normal | Normal | Often normal thyroid function |
| Very low | FT4/FT3 normal | Subclinical hyperthyroidism pattern |
| Low | FT4 or FT3 high | Hyperthyroidism pattern |
For non-pregnant adults with a subclinical hypothyroid pattern, Exeter guidance commonly recommends repeat testing after 3–6 months to confirm persistence before treatment decisions.
Why pregnancy is different
Pregnancy changes thyroid physiology. The American Thyroid Association published updated 2026 guidance for thyroid disease in preconception, pregnancy and postpartum care. Pregnancy-specific clinical interpretation is needed rather than self-diagnosis with a non-pregnant range.
NHS also notes that levothyroxine requirements can change during pregnancy.
Before a thyroid test
- Tell the clinician or laboratory if you take biotin; it can interfere with thyroid tests.
- Mention medicines such as amiodarone, lithium and iodine-containing products.
- Do not stop medicines on your own before testing.
How levothyroxine is usually taken
NHS advises taking levothyroxine on an empty stomach, 30–60 minutes before the first meal of the day.
| Issue | Practical rule |
|---|---|
| Breakfast | Keep a 30–60 minute gap |
| Calcium | Keep at least a 4-hour gap from calcium carbonate |
| Iron / antacids | Avoid taking at the same time |
| Missed dose | Follow the product or clinician instructions; do not double a dose unless specifically told |
| Follow-up | TSH is commonly rechecked after at least 6 weeks following a dose change |
Bangladesh context
A 2019 Bangladeshi study of selected occupational groups found hypothyroidism in 7.0% overall, 8.3% of women and 12.9% of people with diabetes. It was not nationally representative, but it shows thyroid dysfunction is clinically relevant in Bangladesh.
Common mistakes
- making decisions from one TSH result alone
- ignoring the laboratory reference range
- using a non-pregnant range during pregnancy
- taking levothyroxine with breakfast
- taking calcium or iron too close to levothyroxine
- not mentioning biotin before testing
Frequently asked questions
What is a normal TSH?
MedlinePlus lists 0.4–4.8 µIU/mL as a common adult range, but laboratories vary.
What does a high TSH mean?
It often fits a hypothyroid pattern; FT4 helps distinguish overt from subclinical patterns.
What does a low TSH mean?
It can fit a hyperthyroid pattern, especially when FT4 or FT3 is high.
Is the same range used in pregnancy?
No. Pregnancy-specific interpretation is needed.
Can biotin affect thyroid tests?
Yes. Tell the clinician or laboratory if you take biotin.
Related reading
Sources
- MedlinePlus — TSH test, reviewed January 2026.
- Exeter Clinical Laboratory — Thyroid function tests.
- NHS — Levothyroxine, reviewed July 2026.
- American Thyroid Association — 2026 pregnancy/preconception/postpartum guideline.
- Sayeed MA et al. — IMC Journal of Medical Science, 2019.
- NIH Office of Dietary Supplements — Calcium Fact Sheet.
This article is for general health information. Thyroid results should be interpreted with your laboratory range, FT4/FT3, symptoms, age and pregnancy status.
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