Biomarker Glossary

TSH

TSH (thyroid stimulating hormone) is the master regulator of your thyroid gland. A TSH blood test is the first line UK test for screening thyroid function, detecting underactive thyroid (hypothyroidism), overactive thyroid (hyperthyroidism), and monitoring thyroid medication.

Also known as: Thyroid Stimulating Hormone, Thyrotropin

TSH is most informative when read alongside related markers like Free T4 and Free T3. A single number rarely tells the whole story. If your reading sits outside the typical range, share the full report with your GP or healthcare professional before drawing conclusions.

What is TSH?

TSH is released by the pituitary gland in the brain. It tells the thyroid (a butterfly shaped gland at the front of the neck) how much T4 and T3 to produce. T4 and T3 are the thyroid hormones that regulate metabolism, energy, body temperature, weight, mood, heart rate and digestion.

When thyroid hormones are low, the pituitary releases more TSH to push the thyroid harder. When thyroid hormones are high, TSH falls. This means TSH moves in the opposite direction to thyroid function: high TSH usually means underactive thyroid, low TSH usually means overactive thyroid.

TSH is the most sensitive thyroid marker and the standard first line test in the UK. If TSH is abnormal, free T4 and free T3 (and sometimes thyroid antibodies) are tested to confirm the diagnosis.

Why test TSH?

To investigate symptoms of fatigue, weight change, temperature intolerance, mood changes or irregular periods.
If you have a family history of thyroid disease or autoimmune conditions.
To monitor thyroid medication (levothyroxine or carbimazole).
Pregnancy planning and during pregnancy (the thyroid demand changes significantly).
As part of a general health screen or biohacking panel.

To investigate symptoms of fatigue, weight change, temperature intolerance, mood changes or irregular periods.

TSH normal range (UK)

Result	What it means
0.4 to 4.0 mIU/L	Normal (standard UK NHS range)
Below 0.4 mIU/L	Hyperthyroidism (overactive). Free T4 and T3 are usually high.
Above 4.0 mIU/L	Hypothyroidism (underactive). Free T4 is usually low or low-normal.
1.0 to 2.5 mIU/L	Often considered optimal by functional medicine practitioners, particularly for symptom management and during pregnancy planning

The NHS standard range is wide (0.4 to 4.0 mIU/L), but levels above 2.5 mIU/L are increasingly considered suboptimal, particularly when symptoms of hypothyroidism are present or when planning pregnancy. Always interpret TSH alongside free T4 and clinical symptoms.

About these ranges. The ranges above are typical UK clinical lab ranges aligned to NHS and Royal College of Pathologists guidance. Your Chxhealth report will show the specific reference range used by our partner lab, Randox, for each marker. Lab ranges vary slightly between providers and assays. Always interpret your results in the context of the range printed on your own report.

What causes high TSH?

Hypothyroidism (underactive thyroid). Most common cause in the UK is Hashimoto's autoimmune thyroiditis.
Severe iodine deficiency (uncommon in the UK)
Post viral or postpartum thyroiditis (often transient)
Pituitary tumour secreting TSH (rare)
Recovery phase after a non thyroid illness
Inadequate thyroid hormone replacement

What causes low TSH?

Hyperthyroidism (overactive thyroid). Most common cause is Graves' autoimmune disease.
Toxic thyroid nodule(s)
Excessive thyroid medication
Subclinical hyperthyroidism
Pituitary disorders (secondary hypothyroidism causes low TSH with low T4)
Severe non thyroid illness
Pregnancy first trimester (mildly low TSH is normal)

Symptoms of high TSH

Fatigue and tiredness
Weight gain or difficulty losing weight
Feeling cold all the time
Constipation
Dry skin and hair
Hair loss
Low mood or depression
Heavy or irregular periods
Slow heart rate
Brain fog and poor concentration

Symptoms of low TSH

Unexplained weight loss
Rapid or irregular heartbeat
Feeling hot or sweating excessively
Anxiety, irritability or tremor
Difficulty sleeping
Frequent bowel movements or diarrhoea
Lighter or absent periods
Bulging eyes (in Graves' disease)
Muscle weakness

How is TSH tested?

TSH is measured from a blood sample. Fasting is not strictly required but the morning is generally preferred for consistency.

If you take levothyroxine, take it after the blood test, not before.

If your TSH is abnormal, free T4 and free T3 (and thyroid antibodies) should be tested to confirm and characterise the problem.

Chxhealth samples are analysed by Randox, a UK laboratory accredited by UKAS. Results return in 3 to 5 working days.

How to support healthy TSH levels

Eat enough iodine: a balanced UK diet with dairy, eggs, fish and (in small amounts) seaweed provides what you need.
Ensure adequate selenium (Brazil nuts, fish, eggs) which supports thyroid hormone conversion.
Manage stress. Chronic high cortisol suppresses thyroid function.
Treat coeliac disease and other autoimmune triggers, which often co exist with autoimmune thyroid disease.
Avoid excess soy or raw cruciferous vegetables only if you have known iodine deficiency. For most people these foods are fine.

These are general lifestyle suggestions. Chxhealth is an information service. For personal medical advice, please speak to a healthcare professional.

Chxhealth panels that test TSH

13 Chxhealth biomarker panels include TSH. Each is analysed by Randox, a UK laboratory accredited by UKAS. Reports are delivered in 3 to 5 working days.

TSH FAQs

What is a normal TSH level?

0.4 to 4.0 mIU/L is the standard NHS range in the UK. Many functional medicine practitioners and pregnancy planning guidance use a tighter range of 1.0 to 2.5 mIU/L as optimal.

Can TSH be normal but I still have thyroid symptoms?

Yes. TSH within range does not always rule out thyroid problems. Free T4, free T3, thyroid antibodies and reverse T3 give a fuller picture, particularly when symptoms persist.

What does subclinical hypothyroidism mean?

TSH is mildly raised (typically 4 to 10 mIU/L) but free T4 is still in range. Whether to treat depends on symptoms, age, antibody status and pregnancy plans. Discuss with your healthcare professional.

Can stress affect TSH?

Yes. Chronic stress and acute illness can suppress TSH temporarily, and chronic low grade stress contributes to autoimmune thyroid disease.

Why do thyroid issues affect women more?

Autoimmune thyroid disease (Hashimoto's and Graves') is 5 to 10 times more common in women than men. The reasons are not fully understood but involve oestrogen, pregnancy and immune differences.

Often tested with TSH

The biomarkers below are commonly investigated alongside TSH because they reveal connected aspects of the same physiological picture:

Free T4: TSH and free T4 together identify most thyroid dysfunction.
Free T3: Adds the active hormone view, helpful in subclinical hypothyroidism.
Ferritin: Often paired in fatigue workups.
Vitamin D: Another common contributor to thyroid-like symptoms.

Choose the right test for your goal

If you are reading about TSH because of a specific health goal, our buying guides walk you through which Chxhealth panels fit:

Sources and further reading

This page is informed by guidance from the NHS, NICE, Royal College of Pathologists and other UK authoritative bodies. For deeper detail or to verify the information, see:

About this page. Last reviewed: 13 May 2026. Next scheduled review: May 2027. This page has not yet been independently reviewed by a clinician. It is written from authoritative UK medical guidance (NHS, NICE, Royal College of Pathologists, peer-reviewed sources) but has not undergone formal clinical sign off.

Important. Chxhealth is a UK information service. We do not diagnose, treat or prescribe. The reference ranges and information on this page are general educational content and should not be used as a substitute for advice from a qualified healthcare professional. For any concerns about your health or results, please speak to your GP or another healthcare professional.