in pictures and a few words
What it is and why it matters
Checking the thyroid starts with one blood test, and others are added as needed. The tests explain what has gone off track and why.
- Reasons to get checked: tiredness, feeling cold or sweating, weight changes
- A racing heart, hair loss, cycle changes, trouble getting pregnant
- It’s also worth checking if you have a disease where the immune system attacks your own body
Honestly. The same complaints can come from low iron in the blood, lack of sleep or stress. That’s why tests look for the cause rather than treat the symptom.
First test: TSH
TSH is the hormone used to check the thyroid. For problems in the gland itself, it is the best single test in most ordinary cases.
- It is released by a small gland in the brain that controls the thyroid
- Too little thyroid hormone — TSH rises; too much — TSH falls
- That’s why TSH changes earlier than the thyroid hormones themselves
Honestly. If the problem is not in the thyroid but in the brain gland that controls it, TSH may be normal.
One abnormal result is not a diagnosis
A single result outside the range is not yet a diagnosis. The doctor looks at a series of results, not at one point.
- The usual TSH range is about 0.4–4.0 mIU/L (0.4–4.0 µIU/mL)
- Each lab has its own limits — check them on your report
- In older people the upper limit of normal is higher
T4 and when T3 is needed
Free T4 is the thyroid’s own main hormone. When TSH is out of range, T4 is checked alongside it: this shows whether the problem is clear-cut or mild.
- TSH high, T4 low — the gland is underworking
- TSH high, T4 normal — a hidden, mild drop in function
- TSH low, T4 high — the gland is working too hard
Honestly. T3 is usually not the first test. It is needed when TSH is low but T4 is normal. And this test is less accurate than the one for T4.
Antibodies point to the cause
Antibodies hint at the cause of the problem. TPO antibodies alone are not a diagnosis, but a reason to keep an eye on TSH.
- To TPO — present almost always in Hashimoto’s, but also found in many other people
- To Tg — without TPO antibodies, hardly linked to thyroid disease; needed after cancer treatment
- To the TSH receptor — healthy people don’t have them; they point to Graves’ disease
Honestly. The highest risk that the thyroid will weaken is when there are both antibodies and a raised TSH.
Antibodies and pregnancy
Pregnancy is a special case: thyroid tests follow their own rules here.
- Without a thyroid problem, antibodies were linked to more miscarriages and preterm births
- With antibodies, TSH is advised right after pregnancy is confirmed and every 4 weeks until mid-pregnancy
- TSH targets in pregnancy differ from those outside pregnancy
Honestly. Taking the hormone just because of antibodies is disputed: in small studies there were fewer miscarriages, but in a large one (women with miscarriages or infertility) no more babies were born.
Ultrasound — when there is a nodule
Ultrasound shows the structure of the thyroid, not how it works. When there is a nodule, it is a must: it helps decide whether the nodule’s cells need checking.
- Needed if a nodule can be felt, the neck has changed shape, or a nodule was found by chance
- For adults without complaints and without increased risk, looking for cancer on ultrasound is not recommended
- Increased risk: radiation, including radioactive fallout, or thyroid cancer in close relatives
Honestly. For adults without complaints and without risk, the benefit of such a search is small at best, while the harm from extra findings and treatment is at least moderate.
How to prepare
Three simple things help make the result more accurate.
- Get tested in the morning, at about the same hour each time — TSH changes over the day
- Biotin from “hair and nails” supplements can distort the tests — discuss a break with your doctor
- Repeat tests in the same lab: ranges differ between analysers
Honestly. How long to pause biotin is the view of the author of one case report: at least 2 days should be enough to avoid a gross error.
Who should see a doctor first
A racing heart, shaky hands, weight loss for no reason, or eyes starting to bulge — see a doctor without delay.
- A neck nodule is growing fast, your voice has changed or it’s hard to swallow — see a doctor soon
- Planning a pregnancy or pregnant — discuss with your doctor whether you need a TSH test
- Taking biotin — discuss a break before the test with your doctor
Important. A racing heart, shaking and weight loss for no reason are possible signs that the thyroid is working too hard.
In one sentence
Checking the thyroid usually starts with TSH, while T4, less often T3, and antibodies explain what has gone off track and why. Ultrasound is needed when there is a nodule. Testing in the morning and in the same lab makes the result more accurate.
Key points
When to get your thyroid checked
Symptoms of a thyroid problem are non-specific: tiredness, feeling cold or sweating, weight change, palpitations, hair loss, menstrual cycle disturbances, difficulty conceiving. Iron deficiency, lack of sleep or stress cause the same complaints, so tests look for the cause rather than treat the symptom. It is also worth getting checked if you have autoimmune diseases.
TSH — the first test
TSH is made by the pituitary gland. When thyroid hormones are lacking, the pituitary raises TSH; when there are too many, it lowers it. That is why TSH responds to shifts earlier than the hormones themselves.
A typical reference range is roughly 0.4–4.0 mIU/L (0.4–4.0 µIU/mL): the European Thyroid Association (ETA) considers a level from 4.0 mIU/L (4.0 µIU/mL) upwards a mild elevation (Pearce SH Eur Thyroid J 2013. PMID 24783053[5]). Each laboratory has its own limits, and in older people the upper limit is higher (Soh SB Ann Lab Med 2019. PMID 30215224[2]).
One abnormal result is not yet a diagnosis. If TSH is raised but free T4 is normal, the ETA advises repeating both tests together with thyroid peroxidase antibodies, preferably after 2–3 months (Pearce SH Eur Thyroid J 2013. PMID 24783053[5]). The doctor assesses a series of results, not a single point.
Free T4 and when triiodothyronine (T3) is needed
Free T4 is the main thyroid hormone in its free form, not bound to blood proteins. In Ukraine and Europe it is measured in pmol/L, in the US in ng/dL: for example, 15.4 pmol/L (1.2 ng/dL). The reference range depends on the analyser — check the limits on your lab report.
Together, TSH and free T4 give the main patterns: TSH raised and T4 low — hypothyroidism; TSH raised and T4 normal — subclinical (hidden) hypothyroidism; TSH low and T4 raised — hyperthyroidism (Soh SB Ann Lab Med 2019. PMID 30215224[2]).
Free T3 is not usually the first test. It is needed when TSH is low but free T4 is normal: this rules out a form of hyperthyroidism in which only T3 is raised. The T3 test is also less accurate than the T4 test (Soh SB Ann Lab Med 2019. PMID 30215224[2]).
Antibodies: what they mean
Antibodies and pregnancy
With normal thyroid function, antibodies were associated with higher odds of miscarriage (3.9 times in cohort studies) and preterm birth (2.07 times) (Thangaratinam S BMJ 2011. PMID 21558126[8]). For pregnant women with normal thyroid function and TPOAb or TgAb, the ATA advises checking TSH as soon as pregnancy is confirmed and then every 4 weeks until mid-pregnancy (Alexander EK Thyroid 2017. PMID 28056690[9]). TSH targets in pregnancy are different — see TSH norms in pregnancy.
Whether to prescribe levothyroxine for antibodies alone is debatable: in two small studies it reduced the miscarriage rate (Thangaratinam S BMJ 2011. PMID 21558126[8]), but in the large TABLET trial (952 women with TPOAb and miscarriages or infertility) 50 µg a day did not increase the proportion of live births: 37.4% vs 37.9% with placebo (Dhillon-Smith RK N Engl J Med 2019. PMID 30907987[10]).
Ultrasound: when it is needed and when it is not
Ultrasound shows the structure of the gland, not how it works. According to the ETA guideline, it is mandatory when there is a nodule: the ultrasound features decide whether a biopsy is needed (Russ G Eur Thyroid J 2017. PMID 29167761[3]).
The US Preventive Services Task Force (USPSTF) does not recommend looking for thyroid cancer with ultrasound in adults without symptoms and without increased risk (radiation exposure, including radioactive fallout, or thyroid cancer in close relatives): the benefit is at most small, while the harm from overdiagnosis and unnecessary treatment is at least moderate (USPSTF JAMA 2017. PMID 28492905[4]).
How to prepare for the tests
Which test shows what
| Test | What it shows | When it is needed |
|---|---|---|
| TSH | The pituitary signal: rises when hormones are lacking, falls when there is an excess | First, when a problem is suspected, and to monitor treatment |
| Free T4 | How much free thyroxine, not bound to proteins, there is | Together with TSH when TSH is out of range or there are symptoms |
| Free T3 | The gland's second hormone | When TSH is low but free T4 is normal |
| TPOAb | An autoimmune process, Hashimoto's thyroiditis | When TSH is raised; in other cases — at the doctor's discretion |
| TgAb | An autoimmune process, less specifically | Mostly after treatment of differentiated thyroid cancer |
| TRAb | Graves' disease | When TSH is low and there are signs of hyperthyroidism; in pregnant women with Graves' disease |
| Ultrasound | The structure of the gland, nodules | When a nodule can be felt or the shape of the neck has changed |
Who should be more careful
- If you develop palpitations, trembling hands, unexplained weight loss, or your eyes start to bulge, see a doctor without delay: these may be signs of hyperthyroidism.
- If a nodule in the neck grows quickly, your voice changes or swallowing becomes difficult, see a doctor soon.
- If you are planning a pregnancy or are already pregnant, discuss with your doctor whether you need a TSH test: it is not recommended for everyone without exception (Alexander EK Thyroid 2017. PMID 28056690[9]).
Summary
Thyroid testing usually starts with TSH; free T4, less often T3, and antibodies explain what has gone wrong and why. Ultrasound is needed when there is a nodule. A morning sample, a break from biotin and the same laboratory make the result more accurate. You can go through your results and draw up a testing plan at a consultation.
This material is for information only and does not replace a consultation with a doctor. Which tests you need and how to interpret them is for your doctor to decide.
References
- Garber JR Thyroid 2012. PMID 22954017
- Soh SB Ann Lab Med 2019. PMID 30215224
- Russ G Eur Thyroid J 2017. PMID 29167761
- USPSTF JAMA 2017. PMID 28492905
- Pearce SH Eur Thyroid J 2013. PMID 24783053
- Vanderpump MP Clin Endocrinol 1995. PMID 7641412
- Kahaly GJ Eur Thyroid J 2018. PMID 30283735
- Thangaratinam S BMJ 2011. PMID 21558126
- Alexander EK Thyroid 2017. PMID 28056690
- Dhillon-Smith RK N Engl J Med 2019. PMID 30907987
- Barbesino G Thyroid 2016. PMID 27043844
Key facts
- Thyroid-stimulating hormone (TSH) is the best single test for detecting primary thyroid disorders in most outpatient settings; with pituitary disease TSH may be normal (Garber JR, Thyroid 2012).
- If TSH is raised but free thyroxine (T4) is normal, the European Thyroid Association advises repeating both tests together with thyroid peroxidase antibodies, preferably after 2–3 months (Pearce SH, Eur Thyroid J 2013).
- In a 20-year follow-up, women with thyroid antibodies and raised TSH had 38 times higher odds of developing hypothyroidism (Vanderpump MP, Clin Endocrinol 1995).
- In women with normal thyroid function, antibodies were associated with 3.9 times higher odds of miscarriage (cohort studies) and 2.07 times higher odds of preterm birth (Thangaratinam S, BMJ 2011).
- The US Preventive Services Task Force does not recommend looking for thyroid cancer, including with ultrasound, in adults without symptoms and without increased risk (radiation exposure, including radioactive fallout, or thyroid cancer in close relatives) (USPSTF, JAMA 2017).
