Thyroid disorders are among the most common endocrine conditions in the UK, affecting an estimated 1 in 20 people, with women around ten times more likely to be affected than men [1]. Yet thyroid problems are frequently missed because the symptoms (fatigue, weight changes, mood disturbance) overlap with many other conditions. A thyroid function blood test can provide important data to help evaluate whether your thyroid is functioning properly, though results should always be interpreted alongside your symptoms and medical history.
What Does Your Thyroid Do?
The thyroid is a butterfly-shaped gland in the front of your neck. It produces two main hormones: thyroxine (T4) and triiodothyronine (T3). These hormones regulate your metabolic rate, influencing heart rate, body temperature, weight, mood, bowel function, and cognitive speed.
The thyroid is controlled by thyroid-stimulating hormone (TSH), produced by the pituitary gland in your brain (itself regulated by the hypothalamus via thyrotropin-releasing hormone). When thyroid hormone levels drop, the pituitary releases more TSH to stimulate the thyroid. When levels are adequate, TSH falls. This feedback loop means TSH is usually the first marker to shift when something goes wrong, often before T4 or T3 levels move outside their reference ranges.
Understanding Your Results
A standard thyroid function test measures TSH and free T4 (fT4). Some panels also include free T3 (fT3). Here's what the key patterns mean:
Underactive thyroid (hypothyroidism): High TSH with low fT4. This is the most common thyroid abnormality. Symptoms may include:
Fatigue and low energy
Unexplained weight gain
Cold intolerance
Constipation
Dry skin and hair thinning
Low mood or difficulty concentrating
NICE guideline NG145 recommends levothyroxine as first-line treatment once the diagnosis is confirmed [2]. Importantly, NICE requires that a TSH level of 10 mU/L or higher is confirmed on two separate occasions, three months apart, before treatment is started. This is because transient TSH elevations resolve on their own in up to half of cases.
Subclinical hypothyroidism: High TSH with normal fT4. A clinical review in the New England Journal of Medicine estimated that subclinical hypothyroidism affects 4 to 10% of the general adult population, with higher prevalence in women and older adults [3]. However, TSH levels rise naturally with age, so an elevated TSH in an older person doesn't necessarily indicate disease. Not everyone with subclinical hypothyroidism needs treatment. The decision depends on the degree of TSH elevation, the presence of symptoms, and whether thyroid antibodies (anti-TPO) are present. NICE recommends monitoring rather than immediate treatment if TSH is below 10 mU/L and the patient has no significant symptoms [2].
Overactive thyroid (hyperthyroidism): Low TSH with high fT4 and/or fT3. Symptoms may include:
Unexplained weight loss
Anxiety, irritability, or tremor
Palpitations
Heat intolerance
Diarrhoea
This requires prompt medical assessment and is usually managed by an endocrinologist.
Subclinical hyperthyroidism: Low TSH with normal fT4 and fT3. As with subclinical hypothyroidism, this warrants monitoring and clinical judgement rather than automatic treatment.
It's worth remembering that reference ranges represent the middle 95% of a healthy population. By statistical definition, 2.5% of perfectly healthy people will have results above the upper limit. A borderline result in an otherwise well person may not indicate any underlying problem.
When TSH Alone Is Not Enough
Many NHS blood tests only check TSH. In the majority of cases, this is a reasonable screening approach. If your TSH is normal, your thyroid is very likely functioning properly. However, there are situations where checking fT4 and fT3 provides important additional information:
If your TSH is borderline or abnormal and you want to understand the full picture
If you are already taking levothyroxine and your dose needs adjusting
If your symptoms strongly suggest thyroid dysfunction but your TSH is normal (rare, but this can indicate central hypothyroidism, a pituitary problem rather than a thyroid problem)
If there is concern about T3 thyrotoxicosis, where T3 is elevated but T4 remains normal
Thyroid antibodies (anti-TPO and anti-thyroglobulin) are useful in certain situations. Their presence indicates autoimmune thyroid disease (Hashimoto's thyroiditis or Graves' disease) and can help predict whether subclinical hypothyroidism is likely to progress.
Factors That Affect Thyroid Results
Several things can influence your thyroid blood test results:
Biotin supplements (commonly found in hair and nail supplements at doses of 5,000 to 10,000 mcg) can interfere with thyroid immunoassays. Standard multivitamin doses (30 to 150 mcg) require at least 48 hours' washout before testing. Higher-dose supplements should be stopped for at least 72 hours. If you're taking prescription-strength biotin (100 mg or more daily, sometimes used for neurological conditions), speak to your doctor about a longer washout period of up to seven days.
Time of day matters. TSH follows a circadian rhythm, peaking in the early morning and falling during the day. Morning samples (before 9am) are standard practice, as this gives the most consistent baseline for comparison. Be aware that early morning testing captures the TSH peak, so a mildly elevated result may normalise later in the day, which is one reason why repeat testing is important before starting treatment.
Pregnancy significantly alters thyroid physiology and requires pregnancy-specific reference ranges.
Medications including amiodarone, lithium, and corticosteroids can affect thyroid function.
Recent illness can cause temporary thyroid abnormalities (non-thyroidal illness syndrome), which is why testing during acute illness should be avoided where possible.
When to See Your GP
If your thyroid function test shows an abnormality, your GP can assess whether treatment or further investigation is needed. You should see your GP if:
Your TSH is above 10 mU/L (this needs confirming with a repeat test three months later before treatment is considered)
Your TSH is suppressed (low), which needs investigation to exclude hyperthyroidism
You have symptoms of thyroid dysfunction that are affecting your daily life
You are pregnant or planning pregnancy and have a known thyroid condition or abnormal results
You have a family history of autoimmune thyroid disease and new symptoms
If your Brooksby results show any abnormality, take your report to your GP. They can arrange confirmatory testing and referral to endocrinology if needed.
How Brooksby Medical Can Help
Brooksby Medical's Thyroid Function Blood Test measures TSH, free T4, and free T3 from a single sample, with every result including a report from a practising GP explaining what your levels mean in clinical context.
If thyroid antibodies are also relevant, for example to investigate possible Hashimoto's thyroiditis, the Advanced Thyroid Blood Test adds thyroid peroxidase and thyroglobulin antibodies to the panel. For symptoms that could involve both thyroid and hormonal imbalance, the Advanced Thyroid and Sex Hormones Check covers both systems in a single test.
References
British Thyroid Foundation. Thyroid disorders: general information. btf-thyroid.org
NICE. Thyroid disease: assessment and management. NG145. 2019 (updated 2024). NICE.org.uk
Peeters RP. Subclinical hypothyroidism. N Engl J Med. 2017;376(26):2556-2565. DOI: 10.1056/NEJMcp1611144
Written by Dr James Coleman, GP and founder of Brooksby Medical. Dr Coleman is a practising General Practitioner who founded Brooksby Medical to give patients direct access to the blood tests and clinical interpretation they need, without waiting lists.
Medically reviewed: March 2026 | Next review due: March 2027
Medical disclaimer: This article is for informational purposes only and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, medical history, and clinical picture. If you have concerns about your health, please consult your GP.
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Medical disclaimer. This article is for informational purposes and does not constitute medical advice. Blood test results should always be interpreted by a qualified healthcare professional in the context of your individual symptoms, history, and clinical picture. If you have concerns about your health, please consult your GP.