In short
- TSH is a pituitary hormone that drives the thyroid. That is why a high TSH means a slow thyroid and a low TSH an overactive one.
- The reference range in adults is roughly 0.4–4.0 mIU/L. In older people the normal is somewhat higher, in pregnancy lower.
- A raised TSH with a normal fT4 is subclinical hypothyroidism. It is often temporary — repeat the test after 6–12 weeks before doing anything.
- If the result is off, the next tests are fT4 (how much hormone the thyroid makes) and anti-TPO (whether it is autoimmune inflammation).
- Biotin in hair-and-nail supplements can badly distort the result — stop it 48 hours before the blood draw.
TSH is the most often measured hormone. It is also the most often misunderstood. This article explains what the number means and when a deviation matters. It also shows when a result is just a swing caused by an illness or a supplement.
What TSH measures
TSH (thyrotropin) is made in the pituitary gland. It tells the thyroid how much hormone to produce. The system works like a thermostat. If the thyroid works too little, the pituitary raises TSH to push it. So a high TSH is a sign of an underactive thyroid. If the thyroid works too much, TSH drops almost to zero.
Because of this feedback loop, TSH is more sensitive than the thyroid hormones themselves. That is why it is the first test. Only when it is off do fT4 and anti-TPO make sense.
Normal values by age
| Group | Reference range (mIU/L) | Note |
|---|---|---|
| Adults 18–64 years | 0.4–4.0 | The range differs slightly between laboratories. |
| 65–79 years | up to about 5.0 | With age the normal value shifts upwards. |
| 80 years and over | up to about 6.0 | A mildly raised TSH in an elderly person often needs no treatment. |
| Pregnancy | lower limits by trimester | Managed by the gynaecologist; its own range applies, not the general one. |
The app uses your age band, not the general range. So the same value of 4.6 mIU/L gets a different assessment in a 35-year-old than in an 82-year-old.
Pregnancy and planning a pregnancy
In pregnancy lower limits apply, because the placenta itself stimulates the thyroid. Untreated hypothyroidism affects fertility and the course of pregnancy. So TSH belongs among the tests before a planned pregnancy. During pregnancy the gynaecologist takes over. Paying for your own test is not needed then, unless they ask for it.
High TSH: what it means
| Result | What it means | What comes next |
|---|---|---|
| TSH 4–10, fT4 normal | Subclinical hypothyroidism. | Repeat after 6–12 weeks with anti-TPO. It often returns to normal on its own. |
| TSH above 10 | Probable hypothyroidism. | See a doctor; managed even with a normal fT4. |
| TSH raised, anti-TPO positive | Autoimmune thyroid inflammation (Hashimoto's). | Regular monitoring; treatment at the doctor's discretion. |
| TSH raised after a recent illness | Temporary swing. | Repeat after recovery, not during it. |
Symptoms of a slow thyroid: tiredness, feeling cold, weight gain, dry skin, constipation, a slow pulse, slowed thinking. None of them is proof by itself. That is why we measure.
Low TSH
A low TSH means the pituitary is not pushing the thyroid, because there is enough hormone or too much. The causes are an overactive thyroid, too high a dose of thyroid medication, thyroid inflammation and temporary states after childbirth. fT4 and fT3 come next. With symptoms (pounding heart, tremor, weight loss, insomnia) you need to see a doctor quickly.
What distorts the result
- Biotin (vitamin B7 in hair-and-nail products) interferes with immunoassays. It can show a falsely low TSH with a falsely high fT4. Stop it 48 hours before. That interval is standard laboratory advice; the FDA warns about the interference but sets no fixed interval.
- Time of the blood draw: TSH is highest at night and early in the morning, lower in the afternoon. For monitoring, always measure at a similar time.
- Thyroid medication: if you are monitoring your dose, take it after the blood draw. Otherwise fT4 will be temporarily higher.
- Acute illness: during a more serious illness the values are often falsely changed. Wait until you recover.
- Changing laboratory: methods differ. To compare over time, stay with the same one.
Which test adds what
TSH is the first step, not the whole picture. Each further test answers its own question, and none is needed until there is a reason for it.
| Test | Which question it answers | When to order it |
|---|---|---|
| TSH | Is the pituitary pushing the thyroid or calming it? | Always first. On its own it is enough if it is in range and there are no symptoms. |
| fT4 | How much hormone does the thyroid make? | Whenever TSH is off. It separates subclinical from true disorder. |
| fT3 | How much active hormone is circulating? | With a low TSH or to monitor therapy. With a normal TSH it rarely changes anything. |
| Anti-TPO | Is the immune system attacking the thyroid? | With a raised TSH, when planning a pregnancy, with a family history. |
| Anti-TG | Complements anti-TPO when that is negative but suspicion remains. | Less often; mainly when the picture is unclear. |
| Ultrasound | What is the structure of the gland, are there nodules? | With a palpable lump or an enlarged thyroid. A blood test cannot see this. |
Ordering all of them at once in a healthy person tells you no more. It only raises the chance of hitting a borderline value that causes worry without consequence.
How to monitor TSH over time
One value is a photo, a series is a film. With the thyroid it is mainly the second that counts.
- After a deviation is found, repeat after 6–12 weeks. Before that the pituitary–thyroid axis has not settled and the comparison is not fair.
- After a change in medication dose the same rule applies: 6–8 weeks. Otherwise you are measuring a temporary state.
- At the same time of day and in the same laboratory. Morning and afternoon values in the same person can differ by as much as a third.
- Not during an acute illness and not in the week after it. Values are systematically distorted then.
- At least 48 hours without biotin, even if you take it only occasionally in a hair or nail product.
Our app saves every report and draws the curve, so you see the direction and not just the latest number.
Iodine and selenium: what holds and what doesn't
In Slovenia table salt has been iodised since 1953. So severe iodine deficiency in the general population is rare. Adding iodine on your own with autoimmune thyroid inflammation can even make things worse. Excess iodine is a risk factor in Hashimoto's, not a solution.
Selenium is essential for converting thyroid hormones. Supplementing it when deficient lowers anti-TPO levels. What is not proven is that this by itself improves thyroid function or removes the need for medication. Selenium also has a narrow safety window. A permanently high intake is harmful.
Honestly: no supplement replaces thyroid medication when it is needed. If you want a baseline, selenium and iodine can be measured — but measure first, supplement second.
Frequently asked questions
What is a normal TSH value?
In adults roughly 0.4–4.0 mIU/L, somewhat higher in older people, lower in pregnancy. The range printed on your own report always applies.
Do I have to fast for TSH?
No, fasting is not needed. If glucose or lipids are measured in the same panel, fasting applies to the whole panel.
My TSH is 4.5 and I feel fine — is this a disease?
Probably not. A mildly raised TSH with a normal fT4 is subclinical hypothyroidism, which is often temporary. Repeat after 6–12 weeks together with anti-TPO before doing anything.
Do I need fT3?
Rarely. fT3 is measured with a low TSH or to monitor therapy. A low fT3 with a normal TSH is usually not hypothyroidism but the body's response to dieting or another illness.
How often should I repeat TSH?
After a deviation is found, repeat after 6–12 weeks. After a change in medication dose, after 6–8 weeks. If all is fine and you have no symptoms, a check every few years is enough.
Should I take iodine or selenium for my thyroid?
Not on your own. Table salt in Slovenia is iodised, and extra iodine can make autoimmune inflammation worse. Selenium lowers anti-TPO when you are deficient, but it does not replace medication. Both can be measured — measure first.
What do positive anti-TPO antibodies mean?
That the immune system recognises the thyroid as foreign. This is the most common cause of a slow thyroid. Positivity alone with a normal TSH does not mean disease, but it does mean monitoring makes sense.
Sources: ETA (European Thyroid Association) — TSH reference range and subclinical hypothyroidism · ATA/AACE — thyroid screening and management, the role of fT3 · ATA 2017 — thyroid in pregnancy · FDA Safety Communication 2019 — biotin and immunoassays · our knowledge base (TSH age bands 18–64, 65–79, 80+).
