TSHmIU/L
Pituitary-thyroid feedback
Also known as: Thyroid Stimulating Hormone
Typical optimal range
0.27–4.2 mIU/L
A typical reference range used in UK laboratories is around 0.27–4.2 mIU/L, though exact ranges vary between labs and should always be interpreted alongside the result slip from your own laboratory. These figures are indicative — the range printed on your own lab report is the definitive one for your results.
Thyroid function testing and the management of thyroid disorders in the UK are guided by NICE guidance (including NG145 on thyroid disease) and supported by NHS laboratory reference frameworks.
What is TSH?
TSH (Thyroid Stimulating Hormone) is produced by the pituitary gland, a small structure at the base of the brain, and acts as the control signal for the thyroid gland in the neck. When the thyroid is underproducing its hormones, the pituitary raises TSH to push the thyroid to work harder; when thyroid hormones are abundant, TSH falls. Measuring TSH in the blood is the most sensitive and widely used first-line test for thyroid function, because it reflects even subtle changes in thyroid output before those hormones themselves move outside their normal range.
What a high TSH means
A raised TSH suggests the pituitary is working hard to stimulate a thyroid that may not be producing enough hormone — a pattern consistent with an underactive thyroid (hypothyroidism). A single elevated result is a prompt for further investigation, not a diagnosis on its own.
- Primary hypothyroidism. The most common cause of a persistently raised TSH in the UK is autoimmune thyroid disease (Hashimoto's thyroiditis), where the immune system gradually damages the thyroid. A GP will usually check thyroid peroxidase (TPO) antibodies alongside TSH to explore this.
- Subclinical hypothyroidism. Some people have a mildly elevated TSH but normal free T4 levels, with few or no symptoms — a state called subclinical hypothyroidism. NICE guidance recommends repeat testing and shared decision-making about treatment, particularly based on the degree of elevation and presence of symptoms.
- Certain medications. Several medicines — including lithium, amiodarone, and some immunotherapy drugs — can suppress thyroid hormone production and drive TSH upward. Your GP will consider your full medication list when interpreting the result.
TSH can be transiently elevated during recovery from non-thyroidal illness or after very recent changes to thyroid medication, so timing and clinical context matter when interpreting any single result.
What a low TSH means
A low TSH suggests the thyroid may be producing more hormone than the body needs, which can point to an overactive thyroid (hyperthyroidism) or, in people already taking levothyroxine, a dose that is slightly too high. Like a raised result, this warrants follow-up rather than concern in isolation.
- Hyperthyroidism. Conditions such as Graves' disease or a toxic nodular goitre cause the thyroid to produce excess hormone independently of pituitary signalling, suppressing TSH. Symptoms may include palpitations, unintentional weight loss, heat intolerance, and anxiety.
- Over-treatment with levothyroxine. People taking thyroid hormone replacement who have a low TSH may simply need a dose adjustment; this is one of the most common reasons for a suppressed TSH in routine testing and is straightforward for a GP to address.
A mildly low TSH in older adults warrants particular attention, as even subclinical hyperthyroidism is associated with increased risk of atrial fibrillation and bone loss.
Should I be worried?
A single out-of-range TSH result is best thought of as useful information rather than a diagnosis. Thyroid function can fluctuate with illness, stress, and even the time of day a sample is taken, so GPs typically repeat the test — usually with free T4 and sometimes T3 — before drawing any firm conclusions. If your TSH is outside the reference range and you have symptoms such as persistent fatigue, unexpected weight change, feeling unusually cold or hot, or palpitations, it is worth booking an appointment to discuss it. Your GP may also check TPO antibodies and, depending on the picture, consider whether any medications or other health conditions could be contributing. Most thyroid abnormalities, once identified, are very manageable.
What actually helps
While thyroid conditions usually require medical management, certain lifestyle and nutritional factors support healthy thyroid function and are worth being aware of.
- Iodine adequacy. The thyroid requires iodine to produce its hormones, and mild iodine deficiency is more common in the UK than many people realise, particularly among those avoiding dairy. Adequate — but not excessive — iodine intake supports normal thyroid hormone synthesis.
- Selenium intake. Selenium is essential for enzymes that convert thyroid hormones into their active form and protect the thyroid gland from oxidative damage. UK soil is relatively selenium-poor, making dietary sources worth considering.
- Stress management. Chronic psychological stress influences the hypothalamic-pituitary axis and can affect thyroid hormone regulation. Sustained stress-reduction strategies — whether that is regular physical activity, mindfulness, or simply prioritising rest — are broadly supportive.
- Consistent, restorative sleep. TSH follows a circadian rhythm and peaks during the night; poor or disrupted sleep can alter thyroid hormone dynamics. Aiming for consistent sleep timing and adequate duration supports the body's hormonal rhythms more generally.
The following foods have good supporting evidence for contributing to the nutrients most relevant to thyroid function.
Dairy (milk, yoghurt)
One of the most reliable dietary sources of iodine in the UK; a single glass of milk can provide a substantial proportion of the daily requirement.
White fish and seafood
Good sources of both iodine and selenium, supporting two key aspects of thyroid hormone production and conversion.
Brazil nuts
Among the richest dietary sources of selenium; even one or two nuts a day can meaningfully contribute to intake, though very high amounts should be avoided.
Eggs
Contain iodine (primarily in the yolk) and contribute selenium, making them a convenient dual-purpose source for thyroid-relevant nutrients.
Seaweed (in moderation)
Extremely rich in iodine, but regular or large quantities can paradoxically disrupt thyroid function; occasional consumption is fine for most people, but it is not recommended as a daily supplement.
Lean meat and poultry
Provide selenium alongside other nutrients that support general metabolic health, which in turn underpins stable thyroid function.
Track your own TSH
Upload a lab PDF and Better Days trends TSH over time, flags it when it drifts, and suggests foods that help.
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Sources & further reading
These are reputable UK health resources for further reading. This page is written to be consistent with their guidance but is not a substitute for them.
For information only — not medical advice, and not individually reviewed by a clinician. Reference ranges vary by lab and individual. Discuss any results with a qualified clinician before making changes.