Understanding Thyroid Test Results: TSH, Free T4, and What's Normal

Your GP has run a thyroid blood test and the results are back — but the report reads like alphabet soup. TSH 4.2. Free T4 12.1. What does any of this mean, and should you be concerned?

Thyroid problems affect around 1 in 20 people in the UK, according to the British Thyroid Foundation, and women are five to ten times more likely to be affected than men. Yet thyroid blood test results remain some of the most confusing numbers patients encounter. Here's a clear, practical guide to understanding exactly what each marker measures and what your results mean.

What Do Thyroid Tests Measure?

Your thyroid is a small, butterfly-shaped gland at the front of your neck. Despite its modest size, it controls your metabolism, energy production, heart rate, body temperature, and mood. It does this by producing two key hormones: thyroxine (T4) and triiodothyronine (T3).

The entire system is regulated by your brain. The pituitary gland at the base of your brain releases thyroid-stimulating hormone (TSH), which tells your thyroid how much T4 and T3 to produce. When thyroid hormone levels drop, TSH rises to push the thyroid harder. When thyroid hormones are abundant, TSH falls back.

This feedback loop is the reason TSH is such a powerful diagnostic tool — it's your brain's own assessment of whether your thyroid is doing its job.

TSH Explained

TSH (thyroid-stimulating hormone) is almost always the first test your doctor orders. It's the most sensitive single marker for thyroid dysfunction.

Think of TSH as a thermostat. When your thyroid output drops (hypothyroidism), TSH goes up — your brain is shouting at the thyroid to work harder. When your thyroid is overproducing (hyperthyroidism), TSH goes down — your brain is telling the thyroid to ease off.

High TSH (typically above 4.0–4.5 mIU/L, though lab ranges vary) suggests your thyroid is underperforming. This is the hallmark of hypothyroidism — an underactive thyroid. Symptoms include fatigue, weight gain, cold intolerance, constipation, dry skin, and brain fog.

Low TSH (typically below 0.4 mIU/L) suggests your thyroid is overproducing hormones. This points towards hyperthyroidism — an overactive thyroid. Symptoms include weight loss, anxiety, tremor, palpitations, heat intolerance, and diarrhoea.

Practical takeaway: TSH is an inverse indicator. High TSH means low thyroid function; low TSH means high thyroid function. It seems counterintuitive, but once you understand the feedback loop, it makes perfect sense.

Free T4 Explained

Free T4 (free thyroxine) measures the amount of unbound, active thyroxine circulating in your blood. T4 is the main hormone your thyroid produces, and the "free" designation means it's not attached to carrier proteins — it's the portion available for your body to use.

Free T4 is essential for confirming what TSH is suggesting. If TSH is elevated, a low Free T4 confirms true hypothyroidism. If TSH is suppressed, a high Free T4 confirms true hyperthyroidism.

The normal range for Free T4 is typically 12–22 pmol/L in most UK laboratories, though the exact range varies slightly between labs.

Free T4 also helps identify subclinical thyroid disease (more on this below) — situations where TSH is abnormal but Free T4 remains within range, indicating early or mild dysfunction that hasn't yet produced obvious symptoms.

Free T3: When It Matters

Free T3 (free triiodothyronine) is the most biologically active thyroid hormone. While T4 is the main hormone your thyroid produces, your body converts T4 into T3 in peripheral tissues — and it's T3 that does most of the metabolic heavy lifting at a cellular level.

Most GPs don't routinely request Free T3, and in many cases TSH and Free T4 are sufficient. However, Free T3 becomes particularly important when:

• T4 is normal but you still have symptoms — poor T4-to-T3 conversion can leave you symptomatic despite adequate T4 levels
• Hyperthyroidism is suspected — some patients have T3 thyrotoxicosis where T3 is elevated but T4 is normal
• You're on T4-only medication (levothyroxine) and still feel unwell — checking T3 can reveal whether conversion is an issue

The normal range for Free T3 is typically 3.1–6.8 pmol/L in most UK laboratories.

Normal Ranges: A Quick Reference

Marker	Normal Range (typical UK lab)	High Suggests	Low Suggests
TSH	0.4–4.0 mIU/L	Underactive thyroid	Overactive thyroid
Free T4	12–22 pmol/L	Overactive thyroid or overmedication	Underactive thyroid
Free T3	3.1–6.8 pmol/L	Overactive thyroid or excess T3	Poor T4-to-T3 conversion

Important: Reference ranges vary between laboratories. Always compare your results to the specific range printed on your report, not to generic ranges found online.

Subclinical Thyroid Disease

This is where things get nuanced — and where many patients feel frustrated.

Subclinical hypothyroidism means your TSH is elevated (typically between 4.0 and 10.0 mIU/L) but your Free T4 is still within normal range. Your brain has detected a problem and is compensating by pushing TSH higher, but the thyroid is still managing to produce enough hormone — for now.

This affects around 5–10% of the UK population, particularly women over 60. NICE guidelines (NG145) recommend:

• TSH above 10 mIU/L: Offer levothyroxine treatment, as progression to overt hypothyroidism is likely
• TSH 4.0–10.0 mIU/L with symptoms: Consider a trial of levothyroxine, then reassess after 3–6 months
• TSH 4.0–10.0 mIU/L without symptoms: Monitor with repeat testing in 6 months rather than treating immediately

Subclinical hyperthyroidism means TSH is suppressed (below 0.4 mIU/L) but Free T4 and Free T3 are still normal. This can be caused by early Graves' disease, thyroid nodules, or excessive thyroid medication dosing.

Practical takeaway: A single mildly abnormal TSH doesn't necessarily mean you need treatment. The trend over time — whether your TSH is stable, rising, or falling — is far more informative than any single reading.

Thyroid Antibodies

If your thyroid function tests are abnormal — or borderline — your doctor may test for thyroid antibodies to identify the underlying cause.

TPO antibodies (thyroid peroxidase antibodies) are the most commonly tested. Elevated TPO antibodies indicate Hashimoto's thyroiditis, an autoimmune condition where your immune system attacks your thyroid. It's the most common cause of hypothyroidism in the UK.

TSI or TRAb antibodies (thyroid-stimulating immunoglobulin or TSH receptor antibodies) indicate Graves' disease, the most common cause of hyperthyroidism.

Knowing whether your thyroid problem is autoimmune matters for prognosis and monitoring. Hashimoto's tends to cause progressive thyroid destruction over time, meaning someone with subclinical hypothyroidism and positive TPO antibodies is more likely to progress to full hypothyroidism than someone without antibodies.

Thyroglobulin antibodies may also be tested in some cases, particularly for monitoring thyroid cancer after treatment.

What to Do if Your Results Are Abnormal

If TSH is high (possible hypothyroidism)

1. Don't panic. A mildly elevated TSH may be transient — illness, medications (such as lithium or amiodarone), and even the time of day can affect results
2. Retest in 6–8 weeks to confirm the finding before starting treatment
3. Request Free T4 if it wasn't included in the initial panel
4. Consider thyroid antibody testing to determine whether Hashimoto's is the cause
5. Discuss treatment — levothyroxine is the standard treatment for confirmed hypothyroidism, and it's highly effective when dosed correctly

If TSH is low (possible hyperthyroidism)

1. Take it seriously. Untreated hyperthyroidism can cause heart rhythm problems, bone loss, and thyroid storm in severe cases
2. Request Free T4 and Free T3 to confirm the diagnosis
3. Referral to an endocrinologist is appropriate for most cases of confirmed hyperthyroidism
4. Treatment options include antithyroid medications (carbimazole), radioactive iodine, or surgery, depending on the cause and severity

If results are borderline

1. Track over time. Retest in 3–6 months to see whether the abnormality is persistent or resolving
2. Keep a symptom diary. Documenting how you feel helps your doctor make treatment decisions when results sit in the grey zone
3. Test comprehensively. A full thyroid panel (TSH, Free T4, Free T3, TPO antibodies) gives a much clearer picture than TSH alone

Practical takeaway: Thyroid management is as much about trend-watching as it is about individual numbers. Regular testing — particularly if you have a known thyroid condition, a family history, or positive antibodies — allows you to catch changes early and adjust treatment before symptoms escalate.

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