The thyroid gland — a small, butterfly-shaped gland at the base of the neck — produces hormones that regulate virtually every metabolic process in the body: heart rate, body temperature, weight, energy levels, mood, and fertility. When the thyroid produces too little hormone (hypothyroidism) or too much (hyperthyroidism), the effects are wide-ranging and often non-specific — fatigue, weight change, hair loss, mood disturbance, palpitations, and menstrual irregularity are all common presentations. Because these symptoms overlap with dozens of other conditions, thyroid disorders are frequently attributed to stress, depression, or ageing before a blood test is requested.
A thyroid function test is a blood test that measures the levels of thyroid-stimulating hormone (TSH) and, where indicated, free thyroxine (Free T4) and free triiodothyronine (Free T3). These three markers, interpreted together, provide a complete picture of thyroid function. A private thyroid function test at Victoria Medical returns results within 24 hours, with a doctor-reviewed written report explaining what each result means in clinical context.
1. What Does a Thyroid Function Test Measure?
The three key markers in a thyroid function test are TSH, Free T4, and Free T3. Each measures a different part of the hypothalamic-pituitary-thyroid axis — the feedback loop that regulates thyroid hormone production. Understanding what each marker measures is essential for interpreting results correctly.
TSH (thyroid-stimulating hormone) is produced by the pituitary gland and acts as the primary regulator of thyroid hormone production. When thyroid hormone levels fall, the pituitary increases TSH output to stimulate the thyroid to produce more. When thyroid hormone levels rise, TSH falls. This amplified feedback response makes TSH an exceptionally sensitive marker — a small change in thyroid hormone levels produces a large, easily measurable change in TSH. A systematic review by Andersen et al. (2022) demonstrated that TSH discriminates between euthyroid and subclinical hypothyroid states with an AUC of 0.999, compared to 0.853 for Free T4 alone[4].
| Marker | What It Measures | Normal Range (Adult) | When It Is Requested |
|---|---|---|---|
| TSH (thyroid-stimulating hormone) | Pituitary signal to the thyroid — the most sensitive indicator of overall thyroid function | 0.4–4.0 mIU/L (lab-dependent) | First-line test for all suspected thyroid dysfunction |
| Free T4 (free thyroxine) | The main hormone produced by the thyroid gland — the inactive precursor to T3 | 9–25 pmol/L (lab-dependent) | When TSH is abnormal; to confirm overt hypo- or hyperthyroidism |
| Free T3 (free triiodothyronine) | The biologically active thyroid hormone — converted from T4 in peripheral tissues | 3.5–7.8 pmol/L (lab-dependent) | Suspected T3 toxicosis; monitoring T3 supplementation; complex cases |
| TPO antibodies (anti-thyroid peroxidase) | Autoantibodies against the thyroid enzyme TPO — marker of autoimmune thyroid disease | < 34 IU/mL (lab-dependent) | Suspected Hashimoto's thyroiditis or Graves' disease; subclinical hypothyroidism |
| TgAb (thyroglobulin antibodies) | Autoantibodies against thyroglobulin — second marker of autoimmune thyroid disease | < 115 IU/mL (lab-dependent) | Suspected autoimmune thyroid disease; thyroid cancer monitoring |
2. How to Interpret Your Thyroid Function Test Results
Interpreting thyroid function tests requires understanding the relationship between TSH and thyroid hormone levels. The pattern of TSH and Free T4 together determines the diagnosis. A review published in JAMA (Chaker et al., 2025) confirmed that overt hypothyroidism is diagnosed by a high TSH combined with a low Free T4, while subclinical hypothyroidism is defined by a raised TSH with a Free T4 still within the normal range[1].
An important caveat is that TSH reference ranges vary between laboratories and are influenced by age, sex, ethnicity, time of day, pregnancy, medications, and concurrent illness[3]. A TSH of 4.5 mIU/L may be within the normal range at one laboratory and flagged as elevated at another. This is why a doctor-reviewed report — rather than a reference-range printout — is essential for correct interpretation.
| TSH | Free T4 | Interpretation | Common Causes | Next Step |
|---|---|---|---|---|
| Normal (0.4–4.0) | Normal | Euthyroid — normal thyroid function | No thyroid disease | No action required |
| High (> 4.0) | Low | Overt hypothyroidism | Hashimoto's, post-thyroiditis, post-surgery, iodine deficiency | Levothyroxine treatment; NICE NG145 |
| High (4.0–10.0) | Normal | Subclinical hypothyroidism | Early Hashimoto's, ageing, medications | Repeat in 3–6 months; check TPO antibodies; consider treatment if symptomatic |
| High (> 10.0) | Normal or Low | Significant subclinical or overt hypothyroidism | Hashimoto's, post-thyroiditis | Treatment usually indicated; specialist review |
| Low (< 0.4) | High | Overt hyperthyroidism (thyrotoxicosis) | Graves' disease, toxic multinodular goitre, thyroiditis | Specialist referral; anti-thyroid medication or radioiodine |
| Low (< 0.4) | Normal | Subclinical hyperthyroidism | Early Graves', toxic adenoma, excess levothyroxine | Repeat; check Free T3; specialist review if persistent |
| Low (< 0.4) | Low | Secondary hypothyroidism (pituitary) | Pituitary adenoma, Sheehan's syndrome, hypopituitarism | Urgent specialist referral; pituitary function tests |
| Normal | Low | Non-thyroidal illness or secondary hypothyroidism | Acute illness, medications, pituitary disease | Repeat when well; consider pituitary assessment |
3. Symptoms That Warrant a Thyroid Function Test
Thyroid disorders produce non-specific symptoms that overlap with many other conditions. A UK primary care study by Werhun and Hamilton (2015) found that the symptoms most strongly associated with an abnormal TSH result were palpitations, hair loss, weight gain, constipation, and diarrhoea — with pregnancy showing the strongest association of all[5]. Fatigue alone — the most commonly cited reason for requesting a thyroid test — had a low diagnostic yield in this study, underscoring the importance of a full clinical assessment alongside blood testing.
Symptoms suggesting hypothyroidism (underactive thyroid)
- Persistent fatigue and low energy (68–83% of patients)
- Unexplained weight gain (24–59%)
- Feeling cold, particularly in the hands and feet
- Dry skin, brittle nails, and hair loss or thinning
- Constipation
- Low mood, depression, or cognitive slowing (45–48%)
- Menstrual irregularities — heavy periods or oligomenorrhoea (~23%)
- Puffiness around the eyes and face
- Slow heart rate (bradycardia)
- Raised cholesterol — hypothyroidism impairs lipid metabolism
Symptoms suggesting hyperthyroidism (overactive thyroid)
- Palpitations, rapid or irregular heartbeat
- Unexplained weight loss despite normal or increased appetite
- Feeling hot, excessive sweating
- Anxiety, irritability, and restlessness
- Tremor of the hands
- Diarrhoea or frequent bowel movements
- Difficulty sleeping
- Muscle weakness, particularly in the upper arms and thighs
- Goitre (visible swelling at the base of the neck)
- Eye changes — stare, lid retraction, or proptosis (Graves' disease)
4. Hashimoto's Thyroiditis: The Most Common Cause of Hypothyroidism
Hashimoto's thyroiditis — an autoimmune condition in which the immune system attacks the thyroid gland — is the cause of primary hypothyroidism in up to 85% of patients in iodine-sufficient countries such as the UK[1]. It is significantly more common in women than men, and its prevalence increases with age. Many patients with Hashimoto's have a normal TSH for years before the thyroid gland is sufficiently damaged to produce overt hypothyroidism — making TPO antibody testing a valuable early marker.
NICE Guideline NG145 recommends measuring TPO antibodies in patients with subclinical hypothyroidism (raised TSH, normal Free T4) to identify those at highest risk of progression to overt hypothyroidism[6]. Patients with elevated TPO antibodies and a TSH above 10 mIU/L should be offered levothyroxine treatment. Those with a TSH between 4 and 10 mIU/L and positive TPO antibodies should be monitored every 6–12 months and offered treatment if they develop symptoms.
5. Standard vs Extended Thyroid Panel: Which Do You Need?
The standard thyroid function test — TSH and Free T4 — is sufficient for most patients presenting with symptoms of thyroid dysfunction. Sheehan (2016) concluded in a comprehensive primary care review that TSH is the best and, in most cases, the only test needed for initial thyroid assessment[2]. An extended panel — adding Free T3, TPO antibodies, and thyroglobulin antibodies — is indicated in specific clinical situations.
| Panel | Tests Included | Indicated When |
|---|---|---|
| Standard Thyroid Function | TSH, Free T4 | Initial assessment of suspected hypo- or hyperthyroidism; routine monitoring on levothyroxine; annual health check |
| Extended Thyroid Panel | TSH, Free T4, Free T3, TPO antibodies, TgAb | Suspected Hashimoto's or Graves' disease; subclinical hypothyroidism with symptoms; unexplained fatigue with normal standard panel; family history of autoimmune thyroid disease |
| Thyroid Monitoring Panel | TSH, Free T4 (± Free T3) | Patients on levothyroxine — 6–8 weeks after dose change, then annually once stable (NICE NG145) |
| Pregnancy Thyroid Panel | TSH, Free T4, TPO antibodies | All pregnant women with symptoms or risk factors; pre-conception assessment in women with known thyroid disease |
From Our Practice
"I see patients every week who have been told their thyroid is 'normal' — and yet their TSH is 3.8, they have elevated TPO antibodies, and they have been exhausted for two years. A TSH within the reference range does not mean your thyroid is functioning optimally. The reference range is a population average, not a target. If your TSH has risen from 1.2 to 3.8 over three years and you have positive TPO antibodies, that is a trajectory worth monitoring — even if the number is technically normal. This is why I always review thyroid results in the context of the patient's symptoms, history, and trend over time, not just the number in isolation."
Dr George Xynopoulos
MD, MRCP, FACC — Consultant Physician & Cardiologist, Victoria Medical
Frequently Asked Questions
Do I need to fast before a thyroid function test?
No. A thyroid function test does not require fasting. You can eat and drink normally before the test. However, if your thyroid test is being done as part of a broader blood panel that includes a lipid profile or HbA1c, fasting for 8–12 hours will be required for those tests. Fasting requirements are confirmed at the time of booking.
What is a normal TSH level?
The standard reference range for TSH in adults is approximately 0.4–4.0 mIU/L, though this varies between laboratories. A TSH within this range is generally considered normal, but interpretation depends on the clinical context, symptoms, trend over time, and whether Free T4 and TPO antibodies are also measured. A TSH at the upper end of the normal range (3.0–4.0) in a symptomatic patient with positive TPO antibodies warrants monitoring.
Can I have a thyroid test without a GP referral?
Yes. You can book a private thyroid function test at Victoria Medical directly, without a GP referral. A standard thyroid panel (TSH, Free T4) or extended panel (TSH, Free T4, Free T3, TPO antibodies) can be arranged at the same appointment. Results are available within 24 hours with a doctor-reviewed written report.
What happens if my TSH is high?
A raised TSH (above 4.0 mIU/L) with a low Free T4 indicates overt hypothyroidism, which is treated with levothyroxine. A raised TSH with a normal Free T4 indicates subclinical hypothyroidism — whether to treat depends on the degree of TSH elevation, the presence of symptoms, TPO antibody status, and age. NICE NG145 recommends treatment for TSH above 10 mIU/L and for symptomatic patients with TSH above 4 mIU/L and positive TPO antibodies.
What is the difference between TSH and T4?
TSH (thyroid-stimulating hormone) is produced by the pituitary gland and acts as the control signal for thyroid hormone production. It is the most sensitive marker of overall thyroid function. Free T4 (thyroxine) is the main hormone produced by the thyroid gland itself. When TSH is abnormal, Free T4 is measured to determine whether the abnormality is subclinical (TSH abnormal, T4 normal) or overt (both abnormal).
How often should I have a thyroid function test?
For patients on levothyroxine, NICE NG145 recommends testing 6–8 weeks after any dose change, and then annually once the TSH is stable within the target range. For patients with subclinical hypothyroidism who are not on treatment, monitoring every 6–12 months is appropriate. For adults with no known thyroid disease, a thyroid test is a reasonable component of a comprehensive health check at 40 and every 2–3 years thereafter, particularly for women and those with a family history of thyroid disease.
Related Guides
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A clinician's guide to the essential blood tests for adults at 40 — including thyroid function, cholesterol, diabetes, vitamin D, and hormones. What the NHS Health Check misses and why your baseline matters.
Private Health Check: What Is Included and Is It Worth It?
A comprehensive private health check includes thyroid function testing as a standard component. Dr Xynopoulos explains what is included in each package and who benefits most.
What Does a Private GP Do? A Complete Guide
Understand what a private GP consultation involves — including reviewing thyroid function test results, managing hypothyroidism, and arranging specialist referrals.
Cholesterol: What Your Numbers Mean and When to See a Doctor
Hypothyroidism raises LDL cholesterol — making thyroid testing an important part of cardiovascular risk assessment. Dr Xynopoulos explains what your cholesterol numbers mean.
References
- [1] Chaker L, Razvi S, Bensenor IM, et al. Hypothyroidism. JAMA. 2025;333(4):327–339. https://consensus.app/papers/details/313239857c9b503c9d92dc6ee87b5108/
- [2] Sheehan MT. Biochemical Testing of the Thyroid: TSH is the Best and, Oftentimes, Only Test Needed – A Review for Primary Care. Clin Med Res. 2016;14(2):83–92. https://consensus.app/papers/details/81e66e7a8be75909887a31bb7bbc00f1/
- [3] Razvi S, Bhana S, Mrabeti S. Challenges in Interpreting Thyroid Stimulating Hormone Results in the Diagnosis of Thyroid Dysfunction. J Thyroid Res. 2019;2019:4106816. https://consensus.app/papers/details/766ec957f71655a1aa2aeb5ec16ef26a/
- [4] Andersen S, Bruun NH, Pedersen KM, Laurberg P. Interpretation of TSH and T4 for diagnosing minor alterations in thyroid function: a comparative analysis of two separate longitudinal cohorts. Thyroid Res. 2022;15(1):4. https://consensus.app/papers/details/c26b45726d0a5dc9b16412c0c4a8021a/
- [5] Werhun A, Hamilton W. Thyroid function testing in primary care: overused and under-evidenced? A study examining which clinical features correspond to an abnormal thyroid function result. Family Practice. 2015;32(4):441–447. https://consensus.app/papers/details/e7f5f10144ca51d091a43de73ad6c5ee/
- [6] National Institute for Health and Care Excellence. Thyroid disease: assessment and management. NICE Guideline NG145. November 2019 (updated 2023). https://www.nice.org.uk/guidance/ng145
Dr George Xynopoulos
MD, MRCP, FACC — Consultant Physician & Cardiologist
Dr Xynopoulos is a Consultant Physician and Cardiologist at Victoria Medical, 170 Vauxhall Bridge Road, London SW1V 1DX. He provides comprehensive blood testing — including thyroid function panels — with same-day results and a doctor-reviewed written report.
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