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.

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.

Underactive thyroid

An underactive thyroid makes too little thyroid hormone, and then everything runs a little slower. You often notice it as tiredness, feeling cold quickly or gaining weight, but those complaints have many other causes too. One value in your blood shows it first: TSH. An underactive thyroid is also called a slow thyroid or hypothyroidism.

Medically reviewed by

Aäron Spapens

Lifestyle physician at Optimize

What are the symptoms of an underactive thyroid?

ComplaintEvidence
Tired and sluggish
Everything runs a little slower.
strong
Feeling cold quickly
strong
Gaining weight
Without eating differently.
strong
Dry skin and dry hair
Sometimes hair loss too.
strong
Constipation
strong
Hoarseness or a thick neck
From an enlarged thyroid.
strong
Slower thinking
Mostly with a clearly underactive thyroid.
fair
Low mood
A modest link.
fair
Aching or stiff muscles
Sometimes the first or only complaint.
fair
Changed periods
fair

How strong is the evidence?

strong: In the Dutch guideline, or confirmed in several controlled trials.
fair: From one controlled trial, an international guideline or a review of research.
limited: The research does not point one way.

Evidence as of October 2026. New research can shift this.

These complaints are also described with iron deficiency, anaemia, a vitamin B12 deficiency, a vitamin D deficiency, low testosterone.

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.

None of these complaints belongs to an underactive thyroid alone, and a measurement explains no complaint: the reading and the diagnosis belong with your GP. The complaint list follows the Dutch GP guideline on thyroid disorders (NHG) and research on hypothyroidism.

See what your TSH means

When is your TSH too high?

Normal:

0.66 to 4.20 mIU/L

Your valuemIU/L

Enter your value as it appears on your results. Range for adults.

0.100.664.2010.00

No value yet? How to get it measured

What do you do about an underactive thyroid?

No tablet, but keep an eye on it

With a raised TSH and a normal free T4. Your thyroid then still works well, and TSH usually returns to normal by itself. Measuring again makes sense if you get complaints that fit an underactive thyroid. If you are pregnant or want to be, discuss it with your GP.

Thyroid hormone as a tablet

If free T4 is also too low. One levothyroxine tablet a day tops up what your thyroid makes too little of. The dose is set by your TSH.

Take your tablet on an empty stomach

Every day. At a fixed time, at least 30 minutes before breakfast. Take iron and calcium tablets at least 2 hours later.

TSH and free T4 measured

Every 6 weeks. Until your complaints are more or less gone. The dose is adjusted along the way.

A check every 3 months

The first year after that. So the dose keeps fitting.

Once a year

After that. Or sooner if your complaints return.

The timings follow the Dutch GP guideline on thyroid disorders (NHG).

What causes an underactive thyroid?

Autoimmune

Most common

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

Rare in the Netherlands

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

How to get an underactive thyroid measured

Every value below has its own page: what is normal, and what your value means.

Which blood values show an underactive thyroid?
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

And 1 value that completes the picture

Which blood values show an underactive thyroid?
Value	What it shows	Points here when
Free T4	The thyroid hormone itself. Still normal in a subclinically underactive thyroid, low in a genuine shortage.	low

The comprehensive test (€229, single test) measures the values on this page.
Every draw is fasted: nothing to eat or drink from midnight, only water. Appointments are therefore in the morning, until 12:00.
You can draw at 600+ locations near you, no referral needed. See the locations
Results within 24 hours of analysis, in the app.

What is an 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; TSH usually returns to normal by itself, and measuring again only makes sense with complaints. 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.

What is an underactive thyroid confused with?

Each with the one value that tells the two apart.

Subclinically underactive thyroid

Distinguishing value: Free T4

TSH is raised but free T4 still normal: the thyroid keeps up under harder steering. TSH usually returns to normal by itself, and a tablet is not needed then.

Overactive thyroid

Distinguishing value: TSH

The mirror image, with restlessness, weight loss and palpitations instead of slowness. TSH then drops below the range instead.

Iron deficiency

Distinguishing value: Ferritin

Tiredness, cold and hair loss fit both. Ferritin points down the iron route, TSH and free T4 down the thyroid one.

Frequently asked questions

That depends on two values. If your TSH is above the band on your report and your free T4 is too low as well, your thyroid makes too little hormone. If only your TSH is raised, that is called subclinically underactive: your thyroid still works well, and TSH usually returns to normal by itself. For someone in their thirties the upper limit of TSH lies between 4.2 and 5.33 mIU/L, depending on the lab.

That your TSH sits above the band while your free T4 is still normal. Your thyroid then delivers enough hormone, only under harder steering. It is common, by no means always gives complaints, and TSH usually returns to normal by itself. That is why you do not get a tablet for it, and measuring again only makes sense if you get complaints. With a pregnancy or a wish to have children that is different.

Sources 9

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
6.Samuels, psychiatric and cognitive manifestations of hypothyroidism, Current Opinion in Endocrinology, Diabetes and Obesity. 2014. pubmed.ncbi.nlm.nih.gov
7.Bode et al., hypothyroidism and clinical depression: meta-analysis, JAMA Psychiatry. 2021. pubmed.ncbi.nlm.nih.gov
8.Sindoni et al., hypothyroid myopathy, Reviews in Endocrine and Metabolic Disorders. 2016. pubmed.ncbi.nlm.nih.gov
9.Krassas et al., thyroid function and human reproductive health, Endocrine Reviews. 2010. pubmed.ncbi.nlm.nih.gov

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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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