Thyroid blood tests — TSH, T3, T4 and antibodies explained

How the thyroid system works, briefly

Your thyroid sits at the base of the neck and produces two main hormones: T4 (thyroxine, the storage/conversion form) and T3 (triiodothyronine, the active form). The brain (specifically the pituitary gland) monitors circulating thyroid hormone and adjusts production via TSH (thyroid stimulating hormone) — if thyroid hormone is low, TSH rises; if high, TSH falls. This feedback loop is the basis for thyroid testing.

T4 is converted to T3 in peripheral tissues (mainly liver, muscle, kidney) by enzymes called deiodinases. Some conditions (selenium deficiency, severe illness, some medications) impair conversion, so you can have normal T4 but low T3 — something TSH alone won't show.

The four main markers and what they tell you

TSH (thyroid stimulating hormone). Pituitary hormone that controls thyroid output. The first-line screening test. Inversely proportional to thyroid hormone — high TSH usually means underactive thyroid (the pituitary is shouting), low TSH usually means overactive (the pituitary is hushing). Sensitive to most thyroid disease.

Free T4 (free thyroxine). The unbound, biologically active fraction of T4 circulating in the blood. Reflects current thyroid output. Falls in hypothyroidism, rises in hyperthyroidism.

Free T3 (free triiodothyronine). The unbound, biologically active fraction of T3 — the actually-doing-the-work hormone. Can be normal in mild hypothyroidism with low T4 (the body upregulates conversion), or low when conversion is impaired despite adequate T4.

Thyroid antibodies.

• TPO (thyroid peroxidase) antibodies. Most common marker of autoimmune thyroid disease (particularly Hashimoto's thyroiditis). Positive in >90% of Hashimoto's patients. Can be positive years before symptoms develop.
• TG (thyroglobulin) antibodies. Similar to TPO, often co-positive. Sometimes the only positive antibody in autoimmune disease.
• TRAb (TSH receptor antibodies). Marker of Graves' disease (autoimmune hyperthyroidism). Positive TRAb with low TSH and high T4/T3 is essentially diagnostic.

When to test the full panel vs TSH alone

The case for adding free T4, free T3, and antibodies to TSH:

• Symptoms suggestive of thyroid disease (fatigue, weight changes, temperature intolerance, mood changes, hair changes, menstrual irregularity) with normal or borderline TSH.
• Family history of autoimmune thyroid disease.
• Personal history of other autoimmune disease (type 1 diabetes, coeliac, vitiligo).
• Postpartum (TSH alone often misses postpartum thyroiditis).
• Established hypothyroidism not responding well to levothyroxine alone (T3 conversion question).
• Pregnancy planning or active pregnancy (tighter ranges apply).
• Suspected subclinical thyroid disease.
• Mild TSH abnormality (3.5–4.5 mIU/L) with symptoms.

Subclinical thyroid disease

Subclinical hypothyroidism: TSH slightly elevated (typically 4.5–10 mIU/L) but free T4 still within range. Common, often asymptomatic, sometimes symptomatic. Treatment decision is nuanced — not all patients benefit from levothyroxine. Positive thyroid antibodies make progression to overt hypothyroidism more likely (about 5% per year).

Subclinical hyperthyroidism: TSH low but free T4 and T3 within range. Less common; needs assessment for atrial fibrillation risk and bone density implications.

NICE NG145 provides specific guidance on subclinical patterns; in practice, the decision involves age, symptoms, antibody status, and cardiovascular risk.

The 'normal range' vs 'optimal' debate

Standard UK lab reference range for TSH is typically 0.4–4.5 mIU/L. This range was derived from population data; some of the population sampled may have had undiagnosed thyroid disease, which pulls the upper end up artificially.

Patient groups and some clinicians argue the 'optimum' TSH is 0.5–2.0 mIU/L, with anything in the 2.5–4.5 range potentially representing subclinical underactivity, especially when accompanied by:

• Persistent symptoms (fatigue, weight gain, low mood, cold intolerance).
• Positive thyroid antibodies.
• Family history.
• Pregnancy planning (tighter ranges apply).

The mainstream NHS position is that within-range TSH doesn't warrant treatment in most patients. Whether you find that position satisfying depends on your symptoms and clinical context. A full panel including antibodies often changes the picture.

Pregnancy considerations

Tighter TSH ranges apply in pregnancy:

• First trimester: TSH ideally 0.1–2.5 mIU/L.
• Second/third trimester: TSH ideally 0.2–3.0 mIU/L.

Pre-conception and pregnancy thyroid testing is worth doing; subclinical hypothyroidism in pregnancy has implications for both maternal health and fetal neurodevelopment. NICE guidelines support proactive monitoring.

Postpartum thyroiditis

Occurs in around 5–10% of women in the year after birth. Typically a transient hyperthyroid phase (3–6 months postpartum) followed by a hypothyroid phase (6–12 months postpartum), then resolution — though some women progress to permanent hypothyroidism.

Often missed because TSH-only testing at a single timepoint can be normal between phases. Symptoms (low mood, fatigue, weight changes) are often attributed to having a new baby. Full panel testing with antibodies is more informative.

Medication interactions worth knowing

• Biotin supplements. Biotin (often in hair and nail supplements) interferes with TSH assays. Pause biotin for 48–72 hours before testing or your result will be artefactually low.
• Levothyroxine. Take after the blood draw on test day, not before. Otherwise free T4 spikes and gives a misleadingly high result.
• Liothyronine (T3). Short half-life — take last dose 12 hours before testing for accurate free T3 measurement.
• Amiodarone, lithium, steroids, oestrogen. All can affect thyroid testing or function. Mention to your prescriber.

What's typically in a private thyroid panel

Different providers offer different panels. Our standard:

• TSH
• Free T4
• Free T3
• TPO antibodies
• TG antibodies

For suspected Graves' or unexplained hyperthyroidism, TRAb can be added.

How this fits with other testing

Thyroid testing often goes alongside:

• Iron / ferritin. Iron deficiency mimics hypothyroid fatigue. See iron guide.
• Vitamin D. Often low alongside autoimmune disease. See vitamin D guide.
• B12. Deficiency overlaps with fatigue and cognitive symptoms.
• Cortisol / adrenal screen. Sometimes relevant in fatigue presentations.
• For women near menopause: alongside menopause hormone panel — see menopause guide.

If results are abnormal

Your pharmacist will annotate results with context and recommended next steps. Common pathways:

• Clearly abnormal TSH/T4. GP referral with summary letter for assessment and treatment initiation.
• Subclinical pattern with symptoms. GP discussion about whether trial of treatment is appropriate; sometimes endocrinology referral.
• Positive antibodies, normal TSH. Watch-and-monitor pathway with annual TSH check.
• Postpartum pattern. Often resolves without treatment; monitoring schedule.
• Hyperthyroid pattern. Urgent GP / endocrinology referral.

The Leicester clinic context

We process thyroid panels through UKAS-accredited labs with 24–48 hour turnaround. Results are reviewed by Mohammed Kolia (Superintendent Pharmacist, GPhC 2073260) before they reach you. Free summary letter to your GP if follow-up is appropriate.

The next step

If you've had a TSH-only result that doesn't fit your symptoms, or you want a full thyroid panel as part of a broader investigation, booking a same-day phlebotomy slot is straightforward. We can also scope which panel makes sense in a free 10-minute consultation.