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Finding the right tests…
Hormonal
T3, or triiodothyronine, is one of two hormones your thyroid gland releases, and it's the more active of the two — most of the T3 in your blood actually comes from T4 being converted in your tissues, not straight from the thyroid. This test measures T3 directly, either the total amount or just the free, unbound portion, and your doctor usually orders it with TSH and T4 rather than alone, since T3 by itself rarely tells the full story. It's particularly useful when TSH suggests an overactive thyroid but T4 is still in range, since some hyperthyroid cases show up in T3 first. A low T3 is common in people who are severely or chronically ill, even without thyroid disease.
This test reports T3 in your blood, either as total T3 (bound plus free) or free T3 (just the active, unbound portion), in nanograms or picograms per decilitre depending on which is ordered. MedlinePlus lists total T3 at roughly 78 to 158 ng/dL and free T3 at roughly 210 to 440 pg/dL in adults, though your lab's range may differ and ranges shift with age in children. It's almost always ordered with TSH, and often T4, rather than alone, since T3 can be misleading without the others for context. A single blood sample is enough, with no special preparation.
Your thyroid releases mostly T4, and most of the T3 in your blood actually comes from T4 being converted in your liver, kidneys, and other tissues rather than released directly by the thyroid. T3 is the more biologically active of the two, meaning it's the form your cells actually use to regulate metabolism.
TSH and T4 are usually the first tests checked when a thyroid problem is suspected. T3 gets added when those results don't fully explain your symptoms — for instance, when TSH is low, suggesting an overactive thyroid, but T4 is still within range. In that situation, a high T3 can confirm a form of hyperthyroidism sometimes called T3 toxicosis, where T3 is the hormone driving the overactivity even though T4 looks normal.
A high T3, especially alongside a low TSH, generally points toward an overactive thyroid — Graves' disease and toxic nodular goitre are common causes. A low T3 more often reflects severe or chronic illness elsewhere in the body, sometimes called sick euthyroid syndrome, rather than a thyroid problem itself, since the body reduces T3 production during serious illness as a protective response. Pregnancy and oral contraceptives can also raise total T3 by increasing the protein T3 binds to, without any real change in thyroid function.
MedlinePlus reports a typical total T3 range of 78 to 158 ng/dL, and a free T3 range of 210 to 440 pg/dL, in adults. Ranges vary by lab and by age, especially in children, so check the range on your own report.
Total T3 measures all the T3 in your blood, including the portion attached to carrier proteins, while free T3 measures only the small unattached fraction that's actually active in your tissues. Your doctor may order either, depending on your situation.
Some cases of an overactive thyroid show up in T3 before T4 does, a pattern called T3 toxicosis, so your doctor may add a T3 test when your symptoms and TSH suggest hyperthyroidism but T4 alone doesn't confirm it.
This page is general information about the test, not medical advice. Your own report's reference ranges apply — please discuss your results with your doctor.
Yes. Severe illness can lower T3 as the body's way of conserving energy, a pattern called sick euthyroid syndrome, without the thyroid gland itself being diseased. Your doctor will usually wait and retest once you've recovered before drawing conclusions.
Yes. Pregnancy and oral contraceptives raise the level of the protein that carries T3 in the blood, which pushes up total T3 without reflecting a real change in thyroid activity. Free T3 is less affected by this.
No special preparation, including fasting, is needed for a T3 blood test.