Thyroid Test Results Explained: TSH, Free T4 and T3

TSH is the main thyroid screening test. A typical adult TSH is about 0.4–4.5 mIU/L. A high TSH often means the thyroid is underactive (hypothyroidism); a low TSH can mean it's overactive (hyperthyroidism). Free T4 and T3 show how much thyroid hormone is actually in your blood, and they confirm what TSH suggests.

Check your numbers with the free Thyroid Test Checker. It explains each result in plain English and runs in your browser, so your results stay private.

How do TSH, T4 and T3 work together?

Your pituitary gland, at the base of the brain, makes thyroid-stimulating hormone (TSH). TSH tells the thyroid gland in your neck to make thyroxine (T4), which the body converts into the more active triiodothyronine (T3). It's a feedback loop:

• When thyroid hormone is low, the pituitary raises TSH to push the gland harder.
• When thyroid hormone is high, the pituitary lowers TSH.

So TSH moves in the opposite direction to the thyroid's output. That's why a high TSH usually points to an underactive thyroid.

What are typical thyroid test ranges?

Test	Typical adult range	Other common units
TSH	About 0.4–4.5 mIU/L	Same as µIU/mL
Free T4 (FT4)	0.8–1.8 ng/dL	About 10–23 pmol/L
Total T4	5.0–12.0 µg/dL
Free T3 (FT3)	2.3–4.2 pg/mL	About 3.5–6.5 pmol/L
Total T3	80–200 ng/dL
Anti-TPO antibodies	Below about 35 IU/mL	Varies by lab

The checker grades TSH like this: below 0.1 very low, 0.1–0.4 low, 0.4–4.5 normal, 4.5–10 mildly raised, and 10 or more high. Ranges differ between labs and are different in pregnancy, so always check your own report.

What do the common patterns mean?

• High TSH, low free T4: an underactive thyroid (hypothyroidism). The most common cause is Hashimoto's thyroiditis, often with raised anti-TPO antibodies. Symptoms can include tiredness, feeling cold, weight gain, constipation and dry skin.
• Mildly high TSH, normal free T4: subclinical hypothyroidism. Doctors usually repeat the test in a few weeks to months before deciding on treatment.
• Low TSH, high free T4 or T3: an overactive thyroid (hyperthyroidism), for example Graves' disease or a nodule. Symptoms can include weight loss, a fast heartbeat, shakiness, heat intolerance and anxiety.
• Low TSH, normal T4 and T3: subclinical hyperthyroidism, or too much thyroid medicine.
• On levothyroxine: TSH is used to adjust your dose. Changes take about 6–8 weeks to show.

Recent illness, pregnancy, some medicines and biotin (vitamin B7) supplements can change results. Tell your doctor about everything you take.

How do I check my thyroid results?

1. Open the Thyroid Test Checker. It opens on the thyroid group.
2. Choose Enter values and type TSH and, if you have them, free T4, T3 and anti-TPO. Or paste the report text, or upload a digital PDF (scans and photos can't be read).
3. Read what each result means, the possible reasons, and the questions to ask your doctor.

Can diet help thyroid problems?

Diet can't replace thyroid medicine, but it helps with energy, weight and cholesterol, which often change with thyroid problems. Get enough iodine from iodised salt in normal amounts, avoid high-dose iodine or kelp supplements unless prescribed, and take levothyroxine on an empty stomach, apart from calcium, iron and coffee. Our Thyroid Diet Plan builds a weekly menu around these rules.

When should I see a doctor?

See your doctor about any abnormal thyroid result. Seek prompt care for a racing or irregular heartbeat, severe shakiness, confusion, or a swelling in the neck that affects breathing or swallowing. If you're pregnant or planning pregnancy, thyroid results need special attention, because targets are different.

Learn more in how to read a blood test report, or check your thyroid results now.