Free T3 pg/mL to pmol/L Converter

Free T3 in two units: the short answer

To convert free T3 (free triiodothyronine) from pg/mL to pmol/L, multiply by 1.536. To convert pmol/L back to pg/mL, divide by 1.536 (or multiply by 0.651). A free T3 of 3.0 pg/mL is therefore about 4.6 pmol/L, and 6.0 pmol/L is about 3.9 pg/mL. Because pg/mL equals ng/L, results in ng/L read the same as pg/mL. The converter above handles both directions and compares your value with a typical adult range.

Free T3 has a reputation for being confusing, and the units are part of the problem: US results usually sit between about 2 and 4.5, while UK and European results sit between about 3 and 7, so the two ranges overlap numerically. A value of "4.0" is mid-range in pg/mL and low-normal in pmol/L. Getting the unit right is therefore essential before deciding whether a result is high, low or normal. This guide explains what free T3 is, when it is useful, how to convert it, and the factors that make it move.

What free T3 is

Triiodothyronine (T3) is the most biologically active thyroid hormone. It binds to thyroid hormone receptors in almost every cell and regulates metabolic rate, heart rate, body temperature, gut motility, muscle function, brain development and much more.

Only about 20% of the body's T3 is produced directly by the thyroid gland. The remaining 80% is made in tissues – mainly the liver, kidneys and muscle – by removing one iodine atom from T4 using enzymes called deiodinases. This conversion is regulated: during illness, fasting and stress, the body shifts towards making reverse T3, an inactive form, and less T3. That is one reason T3 levels fall in sick people even when their thyroid is healthy.

As with T4, almost all T3 in the blood is bound to proteins, and only about 0.3% is free. Free T3 is the fraction available to tissues. Total T3 (reported in ng/dL or nmol/L) includes bound hormone and has much higher numbers.

pg/mL and pmol/L explained

• pg/mL (picograms per milliliter) – the usual US unit for free T3; identical to ng/L.
• pmol/L (picomoles per liter) – the SI unit used in the UK, Europe, Canada, Australia and elsewhere.

Some US laboratories report free T3 in pg/dL; 1 pg/mL = 100 pg/dL, so 320 pg/dL is 3.2 pg/mL.

Why the factor is 1.536

Triiodothyronine has a molar mass of 651.0 g/mol. A concentration of 1 pg/mL is 1,000 pg per liter, which is 1,000 ÷ 651.0 = 1.536 pmol/L. Conversely, 1 pmol/L = 0.651 pg/mL.

How to convert free T3 step by step

1. Confirm the test is free T3 (not total T3 or reverse T3) and note the unit.
2. If it is in pg/dL, divide by 100 to get pg/mL.
3. For pg/mL to pmol/L, multiply by 1.536.
4. For pmol/L to pg/mL, divide by 1.536.
5. Round to one decimal place.

Worked examples

• 2.3 pg/mL × 1.536 = 3.5 pmol/L – the lower end of a typical adult range.
• 3.2 pg/mL × 1.536 = 4.9 pmol/L.
• 4.2 pg/mL × 1.536 = 6.5 pmol/L – the upper end of a typical adult range.
• 8.0 pg/mL × 1.536 = 12.3 pmol/L – a clearly raised level, typical of active hyperthyroidism.
• 4.0 pmol/L ÷ 1.536 = 2.6 pg/mL.
• 5.5 pmol/L ÷ 1.536 = 3.6 pg/mL.
• 350 pg/dL = 3.5 pg/mL × 1.536 = 5.4 pmol/L.

Mental shortcut

Multiply pg/mL by 1.5 (and add a little), or multiply pmol/L by two-thirds. For 3.6 pg/mL: 3.6 × 1.5 = 5.4, plus a little = 5.5 pmol/L (exact: 5.53).

Free T3 conversion chart

Normal free T3 ranges

A typical adult reference range is about 2.3–4.2 pg/mL (3.5–6.5 pmol/L). Ranges differ considerably by laboratory: some US ranges extend to about 4.4 pg/mL, and many UK ranges run from about 3.1 to 6.8 pmol/L. Use your own laboratory's range.

Children and adolescents

Free T3 is higher in children and teenagers than in adults, reflecting growth and higher metabolic rates, and it declines gradually towards adult levels after puberty. A value just above an adult range may be normal in a teenager, so paediatric ranges must be used.

Older adults

Free T3 tends to decline slightly with age, partly because of reduced conversion from T4 and the higher prevalence of chronic illness. A mildly low free T3 in an older person with a normal TSH is common and rarely needs treatment.

When free T3 is useful

Free T3 is not a routine first-line test. TSH is the best screening test, with free T4 added when TSH is abnormal. Free T3 is mainly helpful in specific situations:

• Suspected hyperthyroidism. When TSH is low, measuring free T3 as well as free T4 helps confirm and grade hyperthyroidism. In T3 toxicosis – seen in early Graves' disease and with some toxic nodules – free T3 is high while free T4 is still normal.
• Monitoring antithyroid treatment. In Graves' disease, free T3 can remain high when free T4 has normalised, signalling that treatment is not yet adequate.
• Amiodarone and other medicines that affect T4-to-T3 conversion.
• Rare disorders such as thyroid hormone resistance or TSH-secreting pituitary tumours.

In hypothyroidism, free T3 is the last hormone to fall, because the body increases conversion of T4 to T3 to compensate. It is therefore not useful for diagnosing hypothyroidism or for routine monitoring of levothyroxine, and major guidelines, including those of the American Thyroid Association and NICE, do not recommend it for those purposes.

Free T3 and levothyroxine or liothyronine treatment

Levothyroxine (T4)

People taking levothyroxine usually have a normal free T3, often in the lower half of the range, while free T4 sits in the upper half. This is because the body relies on conversion of T4 to T3 rather than direct thyroid production. A small group of people continue to have symptoms despite a normal TSH on levothyroxine, and this has driven interest in measuring free T3 and in combination therapy.

Liothyronine (T3) and combination therapy

Liothyronine is synthetic T3. Combination treatment with levothyroxine plus liothyronine has been studied in many trials, without consistent benefit for the average patient. Current guidance – for example from the European Thyroid Association and a 2021 joint ATA/BTA/ETA consensus – allows a carefully monitored trial of combination therapy in selected patients who remain symptomatic despite optimised levothyroxine, after other causes of symptoms are excluded. When T3 is taken, free T3 peaks within two to four hours of a dose and then falls, so blood test timing strongly affects the result; clinicians often monitor TSH and take samples before the morning dose.

High free T3

A raised free T3 is most often caused by:

• Graves' disease – an autoimmune condition, often with eye changes and a smooth enlarged thyroid.
• Toxic nodular goitre or a toxic adenoma – overactive nodules, more common in older people and iodine-deficient regions.
• Thyroiditis – inflammation that releases stored hormone, usually temporarily.
• Excess thyroid hormone medication, especially liothyronine or desiccated thyroid extract.
• Assay interference, for example from biotin or antibodies.

In hyperthyroidism, a free T3-to-free T4 ratio that is particularly high suggests Graves' disease or toxic nodules rather than thyroiditis, although this is only a supporting clue.

Low free T3

A low free T3 with a normal TSH and free T4 is very common in people who are acutely or chronically unwell and is called non-thyroidal illness syndrome or "sick euthyroid syndrome". It is seen after surgery, during infection or heart failure, in kidney and liver disease, during fasting and with eating disorders. It reflects reduced conversion of T4 to T3 and is generally considered an adaptive response; treating it with thyroid hormone has not been shown to help. Low free T3 is also seen in severe or untreated hypothyroidism, together with a high TSH and low free T4.

Free T3, free T4 and TSH together

Free T3 is most informative when read as part of the full thyroid panel. Common combinations include:

These patterns are guides only. Results must be interpreted with symptoms, medicines, the timing of any doses and, where needed, antibody tests and imaging.

Desiccated thyroid extract

Desiccated (natural) thyroid extract, made from pig thyroid glands, contains both T4 and T3 in a ratio of roughly 4:1 – much richer in T3 than the human thyroid's output of about 14:1. As a result, people taking it often have a free T3 at or above the top of the range a few hours after a dose, together with a lower free T4. Major endocrine societies do not recommend desiccated thyroid as first-line treatment because of this non-physiological ratio and variation in content between batches, although some patients take it. If you take it, ask your clinician how to time blood tests relative to your dose.

Free T3 in hyperthyroidism treatment

When antithyroid drugs such as methimazole or carbimazole are started for Graves' disease, free T4 often normalises within four to eight weeks, while free T3 may stay high for longer. Persistently raised free T3 despite a normal free T4 suggests that the dose is not yet sufficient, and it is also associated with a lower chance of long-term remission. After radioactive iodine or surgery, free T3 helps track the transition from hyperthyroidism to normal function or to hypothyroidism.

What affects free T3 results

• Acute and chronic illness, calorie restriction and fasting lower free T3.
• Liothyronine and desiccated thyroid raise free T3 sharply for several hours after a dose.
• Medicines – amiodarone, high-dose glucocorticoids, propranolol (at high doses) and some iodine-containing contrast agents reduce conversion of T4 to T3.
• Biotin interferes with many immunoassays and can produce misleading thyroid results; stop it for at least two days before testing.
• Pregnancy – free T3 assays, like free T4 assays, may be less reliable in pregnancy because of changes in binding proteins.
• Heterophile antibodies and T3 autoantibodies can cause falsely high results; switching assay or using a different method can clarify.

Comparing free T3 results across laboratories

Free T3 assays vary between manufacturers, and reference ranges are tailored to each assay. A result can sit near the top of one laboratory's range and near the middle of another's. When you have results from two countries, convert them into one unit and then look at each result's position within its own range. For example, 4.0 pg/mL at a laboratory with a range of 2.0–4.4 pg/mL is in the upper third of that range, while 6.1 pmol/L (4.0 pg/mL converted) at a laboratory with a range of 3.1–6.8 pmol/L is also in the upper part of the range. The conclusion is similar, which is reassuring; when the conclusions differ, TSH is usually the better guide.

Symptoms and free T3

Because free T3 is the active hormone, it might seem that it should match how people feel. In practice the relationship is loose. Many people with symptoms such as fatigue, weight gain or brain fog have entirely normal thyroid tests, and these symptoms are common in the general population, with many other causes including sleep problems, anaemia, depression and other hormonal conditions. Conversely, older people with hyperthyroidism may have few typical symptoms ("apathetic hyperthyroidism") and present only with weight loss or atrial fibrillation. Thyroid tests should be interpreted alongside a full clinical assessment.

When to see a doctor

See a doctor if your free T3 is above the reference range or if your TSH is outside the range, especially with symptoms of an overactive thyroid such as palpitations, weight loss, tremor, heat intolerance, anxiety, frequent bowel movements or changes in the eyes. Seek urgent care for a very fast or irregular heartbeat, chest pain, fever with confusion or severe agitation in someone with hyperthyroidism. If you take liothyronine or thyroid extract and develop palpitations or chest pain, contact your clinician promptly.