What is underactive thyroid?
The thyroid makes the hormone that sets the pace of virtually every organ. The pituitary in the brain guards that with TSH: if thyroid hormone falls, TSH rises to spur the gland on harder. That makes TSH the most sensitive measurement there is; it moves before thyroid hormone itself falls outside the range. That gives two stages. In a subclinically underactive thyroid only TSH is raised and the gland still keeps up the work under harder steering; that recovers by itself in a share of cases and often only calls for a repeat after a few months. Only when free T4 also falls below the range does the thyroid really deliver too little. The complaints are sometimes there before the second step, but every one of them is non-specific: the same sluggishness, cold and tiredness also belong with a deficiency of iron or vitamin B12, with lack of sleep and with a winter without daylight. The most common cause in the Netherlands is Hashimoto's disease, a smouldering inflammation in which the immune system slowly breaks down the gland; the TPO antibodies show that. After that come an earlier treatment of an overactive thyroid, medicines such as lithium and amiodarone, and a thyroid inflammation after childbirth, which usually passes by itself.
Which blood values show underactive thyroid?
Every value below has its own page with the reference ranges of four Dutch labs.
| Value | What it shows | Points here when |
|---|---|---|
| TSHdefining | The steering hormone from the brain. Rises as soon as the thyroid falls behind: the first and most sensitive value. | high |
| Free T4 | The thyroid hormone itself. Still normal in a subclinically underactive thyroid, low in a genuine shortage. | low |
What are the symptoms of underactive thyroid?
These complaints are described for underactive thyroid in guidelines and research. None of them is exclusive to it: the same complaints fit many other explanations, and an abnormal value can exist without any of them.
A subclinically underactive thyroid, only a raised TSH, often gives no or hardly any complaints. And the complaints that are there are each of them non-specific: every one is also common without a thyroid problem.
- Tiredness, everything a gear slowerin the guideline
Also described at iron deficiency, anaemia, vitamin B12 deficiency, low testosterone.
- Feeling cold sooner than othersin the guideline
- Gaining weight without eating differentlyin the guideline
- Dry skin and dull, brittle hairin the guideline
- Constipationin the guideline
- Slower thinking and forgetfulnessin the guideline
Also described at iron deficiency, vitamin B12 deficiency, low testosterone.
- Low moodin the guideline
Also described at vitamin B12 deficiency, low testosterone.
- Muscle aches or stiffnessin the guideline
Also described at vitamin D deficiency, low testosterone.
- Heavier menstruationin the guideline
- Hoarseness or a thicker feeling in the neckin the guideline
A complaint points in more than one direction, and a measurement explains no complaint: blood testing shows where your values stand, the reading and the diagnosis belong with your GP. The complaint list follows the Dutch GP guideline on thyroid disease (NHG).
What causes underactive thyroid?
Autoimmune
The immune system attacks the thyroid itself, which slowly makes it produce less. In the Netherlands by far the most common route.
- Hashimoto's thyroiditis
- After childbirth, often temporary
Treatment and medication
The thyroid has been reduced, or a medicine slows it down.
- After thyroid surgery or treatment with radioactive iodine
- Medicines such as lithium or amiodarone
Predisposition and iodine
The largest cause worldwide, rare in the Netherlands.
- Iodine deficiency, rare here thanks to iodised bread and salt
- Congenital, or driven from the pituitary, rare
What is underactive thyroid confused with?
Each with the one value that tells the two apart.
Subclinically underactive thyroid
TSH is raised but free T4 still normal: the thyroid keeps up under harder steering. This result is usually measured again after a few weeks, because it regularly recovers by itself.
Distinguishing value: Free T4
Overactive thyroid
The mirror image, with restlessness, weight loss and palpitations instead of slowness. TSH then drops below the range instead.
Distinguishing value: TSH
Iron deficiency
Tiredness, cold and hair loss fit both. Ferritin points down the iron route, TSH and free T4 down the thyroid one.
Distinguishing value: Ferritin
What you do about underactive thyroid
With a slightly raised TSH and a normal free T4 the first thing you do is wait and measure again, after eight to twelve weeks, because a share recovers by itself, and an infection, lack of sleep or a night of poor sleep lifts TSH temporarily. Have blood drawn in the morning, fasting and without your tablet if you already use one, because TSH fluctuates over the day. Anyone who takes in too little iodine, and that is rare in the Netherlands because bakers' salt is iodised, catches up with bread, dairy, fish and seaweed; taking extra iodine does not help and in Hashimoto can fuel the inflammation. If the thyroid is really underactive, the treatment is one tablet of levothyroxine a day, the hormone itself, in a dose adjusted to TSH; it takes six to eight weeks before a new dose shows in TSH, and the complaints settle in that same time. What you do yourself beside that tablet: take it every morning on an empty stomach and wait half an hour with food, coffee, calcium and iron, because those inhibit absorption; exercise regularly, because a slow body only gets slower from sitting still; and enough sleep, because tiredness with an underactive thyroid rarely comes from the thyroid alone. Have TSH measured every year once you are on a fixed dose, and earlier with pregnancy, a weight change of more than a few kilos or returning complaints.
From which TSH does a lab call it high?
This is what the four largest Dutch labs publish themselves, side by side.
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 group per lab
| Lab | 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 |
How to get underactive thyroid measured
- Fasting
- No fasting needed. Eating makes no difference to TSH or free T4.
- Material
- Venous blood, drawn at one of our collection points.
- Results
- Results within 24 hours of analysis, in the app.
- Referral
- No referral from a GP required.
- Which test
- The comprehensive test (€229, single test) measures the values on this page.
- Where to draw
- You can draw at 250+ locations near you, no referral needed. See the locations
Frequently asked questions
From which TSH is your thyroid underactive?
Above 10 mIU/L almost certainly, and below that it depends on free T4 and on the lab: for someone in their thirties the upper limit lies between 4.2 and 5.33 mIU/L, so the same 5 is raised at one lab and normal at another. In that grey zone the gland is subclinically underactive, and then a repeat after a few months is the first step, not a tablet. If free T4 falls too, the thyroid really makes too little.
What is the difference between TSH and free T4?
TSH is the steering hormone from the brain, free T4 the thyroid hormone itself. They move in opposite directions: if the thyroid does too little, TSH goes up to steer harder, and that response is larger than the deficit itself. That is why TSH is the most sensitive first measurement, and free T4 then says how far the deficit really goes and whether treatment is needed.
What does subclinically underactive mean?
That TSH sits above the band while free T4 is still normal. The thyroid then still delivers enough, but only under harder steering. It is common, by no means always gives complaints, and recovers by itself in a share of cases; that is why the result is measured again after eight to twelve weeks. Treatment only comes into view with a TSH above 10, with complaints, when wanting a child or with high TPO antibodies.
Can an underactive thyroid explain weight gain?
In part. An underactive thyroid slows metabolism and retains fluid, which usually makes a few kilos of difference. Large weight gain rarely has the thyroid as its only explanation; sleep, exercise and what you eat weigh more. Conversely treatment makes losing weight easier, but the weight rarely disappears by itself through it, and more levothyroxine than TSH asks for is not a slimming aid.
What about the thyroid in pregnancy?
The need for thyroid hormone rises early in pregnancy, because the child runs on the mother's hormone for the first months, and the normal TSH limits are then lower. Anyone using levothyroxine usually needs a higher dose in pregnancy and is checked more often. The bands on this page apply outside pregnancy; in that period your midwife or doctor reads the result with other limits.
Sources
- 1.NHG-Standaard Schildklieraandoeningen. 2025. richtlijnen.nhg.org
- 2.Unilabs test directory, TSH. 2026. bepalingenklapper.nl
- 3.Star-shl test directory, TSH. 2026. star-shl.nl
- 4.Certe test directory, TSH. 2026. bepalingenwijzer.certe.nl
- 5.Clinical Diagnostics test guide, TSH. 2026. clinicaldiagnostics.nl
This page explains what a result means. It is not a diagnosis and not treatment advice. Discuss complaints or an abnormal value with your doctor.
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