Total T3 (triiodothyronine)
Sum of bound and free thyroid hormone T3 in the blood
Also known as
Definition
Triiodothyronine (T3) is one of the two thyroid hormones, alongside thyroxine (T4); together they control how your body uses energy. Most of the T3 in the blood is bound to transport proteins, and only a small share is free and enters the tissues. Total T3 measures both parts together, free T3 only the unbound part. According to MedlinePlus, total T3 is considered the more accurate of the two measurements.
| Parameter | Value |
|---|---|
| Unit | ng/dl (nmol/l) |
| Reference Range | Typical for adults: 78–158 ng/dl (1.2–2.4 nmol/l). Under age 20, age-specific values apply. Laboratories vary; the reference range on your own report is what counts, and your doctor interprets it. |
Agree on your personal target range with your doctor.
Conversion: 1 ng/dl equals about 0.0154 nmol/l.
↓ What a low value means
A low value can be linked to an underactive thyroid, for example with thyroiditis such as Hashimoto's disease, but also to severe short-term or long-term illness and starvation. Medicines such as amiodarone, lithium, phenytoin, propranolol, antithyroid drugs, androgens or anabolic steroids can lower T3. T3 is not well suited to detecting an underactive thyroid, however, because other thyroid values become abnormal earlier.
↑ What a high value means
A high value can be linked to an overactive thyroid, for example with Graves' disease or toxic nodular goiter, rarely with isolated T3 thyrotoxicosis. A common cause is taking thyroid hormone medicine or certain supplements. Liver disease, pregnancy, birth control pills, estrogens, methadone and some herbal remedies can also raise T3. The value is usually assessed together with TSH and free T4.
When to test
T3 is mainly measured when an overactive thyroid is suspected, for example with a racing heart, weight loss, shaky hands or heat sensitivity, when TSH or T4 results are abnormal or unclear, and to monitor thyroid treatment. Special preparation is usually not needed. Mention the medicines and supplements you take, but do not stop anything without talking to your doctor.
Frequently asked questions
What is the difference between total T3 and free T3? +
Total T3 measures bound and free T3 together, free T3 only the unbound share that enters the tissues. According to MedlinePlus and the American Thyroid Association, free T3 measurements are less reliable than total T3.
Why is T3 rarely used to check for an underactive thyroid? +
With an underactive thyroid, T3 is often the last value to become abnormal. TSH and free T4 show an underactive thyroid earlier. T3 is most informative when an overactive thyroid is suspected.
How is T3 formed in the body? +
The thyroid releases mostly T4. Removing one iodine atom turns it into T3, mainly in the liver and in tissues such as the brain. Selenium deficiency can reduce this conversion; according to MedlinePlus, it is not clear whether this leads to below-normal T3 levels in people.
Why is T3 measured together with TSH and T4? +
Only the combination gives a picture. High T3 and free T4 with low TSH can point to a cause in the thyroid gland, with normal or high TSH to a cause in the pituitary gland. Your doctor interprets the results.
Total T3 (triiodothyronine)
Sources
Last updated: October 05, 2026
This information is for orientation only and does not replace medical advice. Reference ranges can vary by laboratory, method and country.