TSH (thyroid-stimulating hormone) is the pituitary's signal to the thyroid. A TSH of 5.5 mU/L is just above the usual upper limit of about 4.0–4.5. If free T4 is normal, this is called subclinical hypothyroidism: the thyroid is working slightly harder to keep hormone levels normal. It is common, often temporary, and in most adults it is rechecked rather than treated straight away.
| Category | Reference range |
|---|---|
| TSH, adults (typical) | about 0.4–4.0 (some labs to 4.5 or 5.0) mU/L (= µIU/mL) |
| Mildly raised | above the range up to 10 mU/L (= µIU/mL) |
| Markedly raised | 10 or more mU/L (= µIU/mL) |
| Free T4 (FT4) | about 9–25 pmol/L (0.7–1.9 ng/dL) mU/L (= µIU/mL) |
Ranges vary by lab and are different in pregnancy and in older adults; use the range on your report.
When the thyroid makes slightly less hormone, the pituitary gland raises TSH to push it harder. TSH is therefore a sensitive early signal: it moves before free T4 falls out of range. A TSH of 5.5 with a normal free T4 means the thyroid is meeting the body's needs, with a little extra push.
The most common long-term cause in the UK and US is autoimmune thyroiditis (Hashimoto's), where antibodies gradually affect the gland. Your doctor may check thyroid peroxidase (TPO) antibodies: if they are positive, the chance of developing a fully underactive thyroid over the years is higher, so monitoring is more regular.
At a TSH around 5 to 6, most people have no symptoms that can be clearly linked to the thyroid. When thyroid hormone does run low, typical signs include tiredness, feeling cold, weight gain, constipation, dry skin, thinning hair, low mood, heavy periods and slowed thinking. These are also very common for other reasons, so they are interpreted alongside the test results rather than on their own.
UK guidance (NICE) suggests repeating TSH and free T4 after about three months, since many mild rises return to normal. If TSH stays above the range but below 10, treatment may be considered in younger adults with symptoms, and is less often needed in older adults. If TSH is 10 or more on two tests, or free T4 is low, treatment is usually discussed. US guidance is broadly similar.
In pregnancy and when trying to conceive, thresholds are lower and decisions are made more quickly, because thyroid hormone matters for the baby's development.
Discuss a TSH of 5.5 with your GP or doctor, who will usually repeat it with free T4 and possibly antibodies. Contact them sooner, or seek urgent care, if:
Written using public information from:
No need to decode the numbers yourself: take a photo of the report and the app explains “TSH 5.5 (high TSH)” and the rest in plain words — what is normal and what deserves attention.
App StoreGoogle PlayWeb versionIt is slightly above the usual range. With a normal free T4 it is called subclinical hypothyroidism. Most often it is rechecked after a few months, and many such results return to normal.
Usually not. It is a mild finding. It matters more in pregnancy, if TPO antibodies are positive, if free T4 is low, or if it keeps rising on repeat tests.
It is at the upper edge of many labs' ranges, so it may or may not be flagged. On its own it rarely needs action outside pregnancy; your doctor may recheck it later.
Not always. If TSH is below 10, guidelines usually recommend repeating the test and considering treatment only in some situations, such as symptoms in younger adults or pregnancy.
Everyday stress is not a recognised cause of a high TSH. Recovery from illness, age, some medicines and biotin supplements are more likely explanations for a mild rise.
This material is for general information only. It is not a medical opinion and does not replace a consultation with a doctor. Reference ranges depend on the laboratory and the method — rely on the range printed on your own report. Your doctor makes the final interpretation.
Ponyatno