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Hormone Test

Thyroid Profile (TSH, T3, T4)

Your thyroid controls your metabolism, energy and weight. Here's what TSH, T3 and T4 values in your report actually mean.

Written and clinically reviewed by Suman Konda, PharmD, Clinical Pharmacist · Based on peer-reviewed sources · Editorial policy · Not medical advice

Last reviewed and updated: · How we check our content

What is a thyroid test?

The thyroid is a butterfly-shaped gland in your neck that produces hormones controlling metabolism, energy, weight, heart rate and mood. A thyroid profile measures TSH (Thyroid Stimulating Hormone), T3 (Triiodothyronine) and T4 (Thyroxine) to check if your thyroid is underactive (hypothyroidism) or overactive (hyperthyroidism). Thyroid disorders, and hypothyroidism in particular, are common everywhere and affect women several times more often than men.

Thyroid test interpretation gridHow each combination of TSH and free T4 maps to a thyroid diagnosis.Reading TSH together with free T4TSH →LowNormalHighFree T4 →HighNormalLowOverthyperthyroidismOveractive thyroidRare patternsAssay issue or T4 excessRare:TSH-secretingtumourOr hormone resistanceSubclinicalhyperthyroidismOften monitoredNormalNo thyroid diseaseSubclinicalhypothyroidismTreat if TSH high orsymptomsCentralhypothyroidismPituitary cause, notthyroidRecovering /non-thyroidalSeen in acute illnessOverthypothyroidismUnderactive thyroidTSH moves opposite to thyroid hormone, so a high TSH means an underactive gland.Subclinical states have an abnormal TSH with a normal free T4.
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How TSH and free T4 combine to give a thyroid diagnosis.
What the grid shows. TSH and free T4 are interpreted together. A high TSH with a low free T4 is overt hypothyroidism; a high TSH with a normal free T4 is subclinical hypothyroidism, which is the commonest abnormal pattern and often needs monitoring rather than treatment. A low TSH with a high free T4 is overt hyperthyroidism, and a low TSH with a normal free T4 is the subclinical form. The one combination that points away from the thyroid itself is a low TSH with a low free T4, which suggests a pituitary cause. The counter-intuitive part is that a high TSH means an underactive gland, because TSH is the signal telling the thyroid to work harder.

Thyroid Normal Range Chart

TestNormal RangeUnit
TSH (Thyroid Stimulating Hormone)0.4 – 4.0mIU/L
Free T4 (FT4)0.8 – 1.8ng/dL
Free T3 (FT3)2.3 – 4.2pg/mL
Total T45.0 – 12.0µg/dL
Total T380 – 200ng/dL
TSH in Pregnancy (1st trimester)0.1 – 2.5mIU/L

What does HIGH TSH mean?

HIGH TSH Above 4.0 mIU/L: Hypothyroidism

High TSH means your thyroid is underactive (hypothyroidism). Your brain is sending more TSH signals trying to push a sluggish thyroid to work harder. Symptoms include weight gain, fatigue, cold intolerance, dry skin, hair loss, constipation and depression. In countries with adequate dietary iodine, autoimmune Hashimoto's thyroiditis is the most common cause. Treatment is a daily thyroid hormone tablet (levothyroxine).

What does LOW TSH mean?

LOW TSH Below 0.4 mIU/L: Hyperthyroidism

Low TSH means your thyroid is overactive (hyperthyroidism): producing too much hormone. Your brain reduces TSH because there's already excess thyroid hormone. Symptoms include weight loss despite good appetite, racing heart, sweating, anxiety, tremors and heat intolerance. Graves' disease is the most common cause. Treatment includes medications, radioactive iodine or surgery.

What does TSH mean in pregnancy?

PREGNANCY TSH target below 2.5 mIU/L

In pregnancy, normal TSH ranges are different and stricter. TSH above 2.5 in the first trimester is considered elevated and needs treatment. Untreated hypothyroidism in pregnancy increases risk of miscarriage, premature birth and developmental problems in the baby. All pregnant women should have TSH checked early in pregnancy.

Subclinical hypothyroidism

If TSH is mildly elevated (4–10 mIU/L) but T3 and T4 are normal, it's called subclinical hypothyroidism. Many people have no symptoms. Your doctor may or may not recommend treatment depending on your TSH level, symptoms, age and whether you are pregnant or planning pregnancy.

How the thyroid tests fit together

A thyroid profile usually reports TSH along with free T4 (and sometimes free T3). TSH is a pituitary hormone that tells the thyroid how hard to work, so it is exquisitely sensitive: when the thyroid is underactive, TSH rises to push it; when overactive, TSH falls.

This is why TSH is the primary screening test, with free T4 confirming the picture. A high TSH with low free T4 confirms an underactive thyroid (hypothyroidism), while a low TSH with high free T4 confirms an overactive thyroid (hyperthyroidism).

Common causes of abnormal thyroid results

The most frequent cause of an underactive thyroid is Hashimoto's thyroiditis, an autoimmune condition confirmed by thyroid antibody (anti-TPO) testing. An overactive thyroid is most often due to Graves' disease, another autoimmune condition.

Thyroid problems are far more common in women and become more likely with age, which is why testing is often done when investigating symptoms such as unexplained weight change, fatigue, cold or heat intolerance, palpitations, or changes in mood and periods.

Subclinical thyroid disease

Sometimes TSH is mildly abnormal while free T4 remains normal: called subclinical hypo- or hyperthyroidism. This does not always need treatment, and the decision depends on how abnormal the TSH is, whether antibodies are present, symptoms, age and pregnancy plans.

Because of this, a borderline thyroid result is often repeated after a few weeks rather than acted on immediately, to see whether it is a temporary fluctuation or a persistent trend.

Frequently Asked Questions

Do I need to fast for a thyroid test?
Fasting is not usually required, but because TSH varies through the day (it is highest in the early morning), consistency in timing helps when monitoring: many clinics prefer a morning sample.
Can medicines affect thyroid results?
Yes. Biotin supplements can interfere with some thyroid assays, and drugs such as steroids, lithium and amiodarone affect thyroid function. Always mention your supplements and medicines.
Why is TSH checked before T4?
Because TSH is the most sensitive early indicator of thyroid dysfunction: it often changes before free T4 moves outside the normal range.
Are thyroid problems lifelong?
Autoimmune hypothyroidism usually requires lifelong hormone replacement, but some thyroid disturbances, such as those after pregnancy or a viral thyroiditis, can be temporary and resolve.

References

The clinical information on this page is based on peer-reviewed sources indexed in PubMed, the biomedical literature database of the US National Library of Medicine.

  1. Koulouri O, Moran C, Halsall D, Chatterjee K, Gurnell M. Pitfalls in the measurement and interpretation of thyroid function tests. Best Pract Res Clin Endocrinol Metab. 2013;27(6):745-62. doi:10.1016/j.beem.2013.10.003 · PMID 24275187
  2. Pearce EN. Management of hypothyroidism and hypothyroxinemia during pregnancy. Endocr Pract. 2022;28(7):711-718. doi:10.1016/j.eprac.2022.05.004 · PMID 35569735
  3. Physiology, Thyroid. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK519566

Related reading

Medical Disclaimer: TSH ranges can vary by laboratory and are different during pregnancy. This information is for educational purposes only. Never start or stop thyroid medication without consulting your endocrinologist or physician.