What is TSH?
Your thyroid makes the hormones that set the pace of your metabolism: how fast your cells burn energy, how warm you feel, how fast your heart beats and how sharp you think. How much it makes is decided by your pituitary, a gland under your brain, with TSH. If the thyroid delivers too little, the pituitary sends more TSH to drive it; if it delivers too much, the pituitary stops sending. TSH therefore moves the opposite way to the thyroid, and that is the first thing to know to read the value. The second is that the pituitary is oversensitive. A small fall in thyroid hormone gives a large rise in TSH, and so TSH responds long before free T4 leaves the band. That makes TSH the more sensitive value of the two, but also the value that often deviates without anything being wrong: after an illness, in recovery, with age, with a lack of sleep, and with biotin from supplements, which disturbs the measurement itself. A TSH between the upper limit and 10 mIU/L with a normal free T4 occurs in one in twenty adults, and in half of them it is normal again on a repeat after a few months. Your report shows TSH in mIU/L. Healthy for most adults sits roughly between 0.4 and 4, though every lab sets the limits differently and they shift up with age. Above 10 mIU/L the thyroid is almost certainly underactive, below 0.1 mIU/L it is overactive or the dose of thyroid hormone too high.
Why is TSH relevant?
A thyroid that goes out of balance does so insidiously, and the symptoms look like ordinary tiredness. An underactive thyroid makes you tired, cold, slow in thought, makes you gain weight, gives a dry skin, constipation and a slow heartbeat; an overactive thyroid makes you restless, warm, makes you lose weight, gives palpitations, trembling hands and poor sleep. Both are common, especially in women, and both are very treatable. TSH is the value that makes the difference between a suspicion and a diagnosis, and it is the reason TSH goes with almost every blood test. For anyone taking thyroid hormone, TSH is the value the dose is tuned on. A TSH that stays too high means the dose is too low or the tablets are not absorbed well, which happens if you take them with coffee, calcium, iron or an acid-reducing medicine. A TSH that becomes suppressed means a dose that is too high, and in the long run that costs bone density and lifts the risk of atrial fibrillation. The goal is a TSH in the band, measured at least six weeks after every adjustment. For anyone who is healthy, TSH is mainly a value not to read too much into. A slightly raised TSH with a normal free T4 and without symptoms is usually not a disease but a snapshot, and in older people a higher TSH is simply age. What does count: the trend over the years, whether there are antibodies, and whether you have symptoms that fit.
TSH decision limits
What counts beside the number:
- Free T4 from the same draw, because TSH says what the brain asks and free T4 what the thyroid delivers
- Your age, because above seventy a higher TSH belongs to you
- Whether you were ill in the weeks before, because recovery lifts TSH temporarily
- Whether you take biotin, and whether you take thyroid hormone with coffee, calcium, iron or an acid-reducing medicine
- Symptoms that fit: tired and cold with a high TSH, restless and palpitations with a low one
- With a raised TSH: TPO antibodies and a repeat after three months
The limits shift with age and in pregnancy, and differ per lab, so read your value against the band on your own report. More important than the band is the free T4 beside it and the repeat: a slightly raised TSH with a normal free T4 corrects by itself in half of people after a few months. TSH is also written as mU/L or µIU/mL; that is the same unit.
What Dutch labs actually use
No two of these labs use the same range. For an eighteen-year-old the lower bounds differ by a factor of 2.85: Star-shl starts at 0.27 and Certe at 0.77. And at two of the four the upper bound climbs with age: Certe gives a ninety-year-old 7.14 where a thirty-year-old gets 4.55.
0.66 – 4.2mIU/L
Between 0.27 and 0.66 and between 4.2 and 5.33 it depends on the lab, because each sets its limits on its own method and population. Above 5.33 every lab calls it abnormal.
every lab calls 0.66 – 4.2 mIU/L normal. Between 0.27 and 0.66 and between 4.2 and 5.33 it depends on the lab.
Every age, per lab
| Laboratory | Group | Reference range |
|---|---|---|
| Unilabs(Saltro, Medlon, SHO, Atalmedial)2026 | 18 to 50 yrs | 0.66–4.71 mIU/L |
| Women 50 to 70 yrs | 0.68–5.51 mIU/L | |
| Men 50 to 70 yrs | 0.66–4.71 mIU/L | |
| Women 70 to 90 yrs | 0.62–6.19 mIU/L | |
| Men 70 to 90 yrs | 0.65–5.63 mIU/L | |
| Women 90+ | 0.46–7.04 mIU/L | |
| Men 90+ | 0.55–5.76 mIU/L | |
| Star-shl2026 | 18 and over | 0.27–4.2 mIU/L |
| Certe2026 | 18 to 20 yrs | 0.77–4.92 mIU/L |
| 20 to 30 yrs | 0.67–4.76 mIU/L | |
| 30 to 40 yrs | 0.63–4.55 mIU/L | |
| 40 to 50 yrs | 0.65–4.93 mIU/L | |
| 50 to 60 yrs | 0.67–5.23 mIU/L | |
| 60 to 70 yrs | 0.72–5.66 mIU/L | |
| 70 to 80 yrs | 0.67–6.21 mIU/L | |
| 80 to 90 yrs | 0.6–6.67 mIU/L | |
| 90 to 100 yrs | 0.54–7.14 mIU/L | |
| Clinical Diagnostics2026 | All adults | 0.38–5.33 mIU/L |
Each band as the lab publishes it, retrieved in 2026, unless marked otherwise.
TSH high or low: what it means
A low TSH means the pituitary is slowing the thyroid because too much hormone is circulating. The most common explanation first: an overactive thyroid, usually Graves' disease or an overactive nodule; too high a dose of thyroid hormone, the most common reason in anyone taking tablets; an inflammation of the thyroid, which temporarily leaks hormone and afterwards often swings to underactive; and biotin from supplements, which disturbs the measurement without the thyroid doing anything. Below 0.1 mIU/L with a high free T4 the thyroid is overactive, and then treatment belongs with it. A low TSH with a normal free T4 is repeated after a few months. A high TSH means the pituitary is driving the thyroid because it delivers too little. An underactive thyroid, usually Hashimoto's, in which the immune system slowly breaks the thyroid down, is the most common reason; with a low free T4 alongside that is the diagnosis, and the treatment is thyroid hormone in tablet form. Age lifts TSH without disease: above seventy the upper limit sits a good deal higher at some of the labs. Recovery after an illness or an operation lets TSH shoot up for a few weeks. Rare are too little or too much iodine. Above 10 mIU/L the thyroid is almost certainly underactive; in between, with a normal free T4, you repeat after three months with antibodies alongside. What you do about it: with a slightly raised TSH without symptoms first nothing but repeating, because half correct by themselves. Make sure of enough iodine, from bread with iodised salt, dairy, fish and eggs, because without iodine the thyroid cannot make hormone; too much iodine from kelp or supplements is just as bad. Selenium from nuts, fish and eggs helps the thyroid convert the hormone. Stop biotin three days before the draw. With excess weight TSH sits slightly higher and falls with the weight. In Hashimoto's thyroid hormone is the treatment, and the dose is tuned on the TSH; with an overactive thyroid, drugs that slow production, radioactive iodine or an operation are the options. Read TSH beside free T4, because TSH says what the brain asks and free T4 what the thyroid delivers, and together they say whether it is about the thyroid or about the pituitary. With a raised TSH, TPO antibodies belong with it, which say whether it is Hashimoto's, and CRP if you have just been ill. An underactive thyroid also lifts LDL.
What lowers it
The pituitary is slowing the thyroid, because too much hormone is circulating or because the measurement thinks so.
| Cause | How often |
|---|---|
| Overactive thyroidUsually Graves' disease, in which antibodies drive the thyroid, or a nodule making hormone on its own; free T4 is then high | Often |
| Too high a dose of thyroid hormoneThe most common reason in anyone taking tablets; a suppressed TSH costs bone density in the long run and lifts the risk of atrial fibrillation | Often |
| Inflammation of the thyroidAn inflamed thyroid temporarily leaks hormone, after a virus or a birth; after weeks it often swings to underactive and usually recovers by itself | Sometimes |
| Biotin in supplementsHigh doses of biotin disturb the measurement and give a falsely low TSH; stop three days before the draw | Sometimes |
What raises it
The pituitary is driving the thyroid, because it delivers too little or because the body just had something else on its mind.
| Cause | How often |
|---|---|
| Underactive thyroidUsually Hashimoto's, in which the immune system slowly breaks the thyroid down; with a low free T4 alongside that is the diagnosis, and TPO antibodies confirm it | Often |
| AgeTSH rises with the years without disease; above seventy the upper limit sits a good deal higher at some of the labs | Often |
| Recovery from illnessAfter an infection, an operation or a hospital stay TSH shoots up for a few weeks; measure only six weeks after recovery | Sometimes |
| Too little or too much iodineWithout iodine the thyroid cannot make hormone, and an excess from kelp or supplements shuts it down instead | Rare |
How does a TSH blood test work?
- Referral
- Not needed. You can have TSH measured yourself, and it goes with almost every blood test; ask for free T4 alongside.
- Fasting
- Not strictly needed, though TSH sits slightly higher fasting in the morning than later in the day; measure at the same moment each time if you want to compare.
- When
- Not during or in the six weeks after an illness, and at least six weeks after every adjustment of thyroid hormone. Stop biotin three days beforehand, and take your thyroid tablet only after the draw.
- The draw
- One tube of blood from a vein in your arm. Free T4, TPO antibodies and CRP come from the same draw.
- Repeating
- With a slightly deviating TSH after three months, because half correct by themselves. Anyone taking thyroid hormone measures six weeks after every dose change and then once a year.
- Which test
- TSH is in the comprehensive test (€229) and in .
- Where to draw
- You can draw at 350+ locations near you, no referral needed. See the locations
Frequently asked questions
The hormone your pituitary, a gland under your brain, uses to steer your thyroid. If the thyroid delivers too little hormone, the pituitary sends more TSH to drive it; if it delivers too much, the pituitary stops sending. So TSH moves the opposite way to the thyroid, and because the pituitary responds so sensitively, TSH responds first.
Related biomarkers
Read on
Sources
- 1.Unilabs, test catalogue (Dutch): TSH. 2026. bepalingenklapper.nl
- 2.Star-shl, test catalogue (Dutch): TSH. 2026. star-shl.nl
- 3.Certe, test catalogue (Dutch): TSH. 2026. bepalingenwijzer.certe.nl
- 4.Clinical Diagnostics, lab guide (Dutch): TSH. 2026. clinicaldiagnostics.nl
- 5.NHG guideline on thyroid disorders (M31), Dutch College of General Practitioners. 2025. richtlijnen.nhg.org
- 6.LESA Laboratory diagnostics (Dutch primary-care laboratory agreement). 2026. richtlijnen.nhg.org
Educational information only, not medical advice. Consult a healthcare professional for clinical decisions.
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