Understanding TSH, Free T4, Free T3, and what your thyroid function test results mean
Written by Trevor Sebuliba, Advanced Nurse Practitioner, BSc, MSc, NMP · Reviewed 2026
Thyroid function tests (TFTs) are among the most commonly ordered blood tests in the UK, with over 40 million thyroid tests carried out globally each year. Yet the result — a TSH number with a reference range — often leaves patients with more questions than answers. This guide explains the thyroid hormone system, what TSH measures, and how to interpret your results.
The thyroid is a butterfly-shaped gland in your neck that produces two key hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate metabolism — they influence how fast your body burns energy, your heart rate, body temperature, digestion, mood, and many other essential functions. Almost every cell in the body has receptors for thyroid hormones.
TSH stands for Thyroid Stimulating Hormone (also called thyrotropin). It is produced by the pituitary gland in the brain — not the thyroid itself. TSH acts as a control signal: when the thyroid is producing too little hormone, the pituitary increases TSH output to stimulate the thyroid to produce more. When the thyroid is producing too much, the pituitary reduces TSH.
This feedback loop means that TSH moves in the opposite direction to thyroid hormone levels: a high TSH indicates low thyroid output (underactive thyroid / hypothyroidism), while a low TSH indicates high thyroid output (overactive thyroid / hyperthyroidism). This inverse relationship is one of the most common sources of confusion when reading results.
| TSH Level (mU/L) | Interpretation |
|---|---|
| Below 0.4 | Low — possible hyperthyroidism or over-treated hypothyroidism |
| 0.4 – 4.5 | Normal range for most adults |
| 4.5 – 10 | Mildly elevated — possible subclinical hypothyroidism |
| Above 10 | Significantly elevated — overt hypothyroidism likely |
Reference ranges vary slightly between laboratories and by age. Older adults naturally tend toward higher TSH values. Pregnant women have different target ranges (typically lower TSH is preferable in the first trimester).
Free T4 (FT4 or fT4) measures the amount of unbound thyroxine in the blood — the portion available to cells. T4 is the main hormone produced by the thyroid. Most T4 circulates bound to proteins and is inactive; only the "free" fraction is biologically active.
A normal FT4 reference range is typically 12 – 22 pmol/L in most UK labs. A low FT4 combined with a high TSH confirms primary hypothyroidism. A high FT4 combined with a low TSH confirms hyperthyroidism.
T3 is the active form of thyroid hormone — it is roughly four times more biologically potent than T4. Most T3 in the body is converted from T4 in peripheral tissues (liver, kidneys, muscles). FT3 is not routinely measured in NHS thyroid screening but may be checked if hyperthyroidism is suspected or if a patient on levothyroxine (T4 replacement) is still symptomatic. A normal FT3 reference range is typically 3.5 – 6.5 pmol/L.
This is a common finding — a mildly raised TSH with a normal FT4. Symptoms are often absent or mild. NICE recommends a repeat test after 3 months to confirm, as TSH can fluctuate. Treatment is not always needed, but your GP may recommend it if you have symptoms, are trying to conceive, or have a TSH above 10.
When TSH is significantly elevated and FT4 is low, the diagnosis is overt hypothyroidism. Common symptoms include fatigue, weight gain, feeling cold, dry skin and hair, constipation, slow heart rate, and low mood. The most common cause in the UK is Hashimoto's thyroiditis (autoimmune thyroid disease). Treatment is levothyroxine (synthetic T4), a once-daily tablet. The dose is adjusted until TSH normalises.
A suppressed TSH with normal thyroid hormones. Often asymptomatic. May be caused by early Graves' disease, a thyroid nodule, excess thyroid medication, or transient thyroiditis. Monitoring is usually recommended.
The thyroid is overproducing hormones. Common symptoms include unintentional weight loss, palpitations, tremor, sweating, heat intolerance, anxiety, and frequent bowel movements. Causes include Graves' disease, toxic multinodular goitre, and thyroiditis. Treatment options include antithyroid drugs (carbimazole in the UK), radioiodine therapy, and surgery.
If your TSH is abnormal, your doctor may request thyroid antibodies — specifically anti-TPO (thyroid peroxidase antibodies) and anti-TG (thyroglobulin antibodies). Positive anti-TPO antibodies are present in around 95% of Hashimoto's thyroiditis cases and about 75% of Graves' disease cases. Their presence indicates an autoimmune cause, which helps guide long-term management.
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