Free T4 ng/dL to pmol/L Converter

Free T4 in two units: the short answer

To convert free T4 (free thyroxine) from ng/dL to pmol/L, multiply by 12.87. To convert pmol/L back to ng/dL, divide by 12.87 (or multiply by 0.0777). A free T4 of 1.2 ng/dL is therefore about 15.4 pmol/L, and 18 pmol/L is about 1.40 ng/dL. The converter above works in both directions and compares your result with a typical adult reference range.

Thyroid results are among the most frequently compared across borders, partly because thyroid conditions are common and long-lasting, and partly because online communities share results in both units. A free T4 of "1.1" and one of "14" can describe the same person on the same day. This guide explains what free T4 is, how to convert it, how it is interpreted together with TSH, what changes it in pregnancy and with medicines, and the most common traps in reading results.

What free T4 measures

The thyroid gland produces mainly thyroxine (T4) and a smaller amount of triiodothyronine (T3). T4 is a prohormone: most of it is converted in tissues such as the liver, kidney and muscle into T3, the more active form.

More than 99.9% of T4 in the blood is bound to carrier proteins – mostly thyroxine-binding globulin (TBG), with smaller amounts bound to transthyretin and albumin. Only about 0.02–0.03% is free. The free fraction is the part available to enter cells, and it is not affected by changes in binding proteins in the way that total T4 is. That is why most laboratories now measure free T4 rather than total T4.

Free T4 is controlled by thyroid-stimulating hormone (TSH) from the pituitary gland. When free T4 falls, TSH rises to stimulate the thyroid; when free T4 rises, TSH falls. This feedback is extremely sensitive: a small change in free T4 causes a large, opposite change in TSH. For this reason TSH is usually the first test, and free T4 is added to clarify the cause of an abnormal TSH or to monitor treatment.

ng/dL and pmol/L explained

• ng/dL (nanograms per deciliter) – the usual unit for free T4 in the United States.
• pmol/L (picomoles per liter) – the SI unit, used in the UK, Europe, Canada, Australia and most other countries.

Do not confuse free T4 with total T4, which is reported in µg/dL or nmol/L and has much higher values (roughly 5–12 µg/dL, 64–154 nmol/L).

Why the factor is 12.87

Thyroxine has a molar mass of 776.9 g/mol. A concentration of 1 ng/dL is 10 ng per liter, or 10,000 pg per liter, which is 10,000 ÷ 776.9 = 12.87 pmol/L. Conversely, 1 pmol/L = 0.0777 ng/dL.

How to convert free T4 step by step

1. Confirm the test is free T4 (not total T4) and check the unit.
2. For ng/dL to pmol/L, multiply by 12.87.
3. For pmol/L to ng/dL, divide by 12.87.
4. Round pmol/L to one decimal place and ng/dL to two decimal places.

Worked examples

• 0.8 ng/dL × 12.87 = 10.3 pmol/L – the lower end of a typical adult range.
• 1.0 ng/dL × 12.87 = 12.9 pmol/L.
• 1.4 ng/dL × 12.87 = 18.0 pmol/L.
• 1.8 ng/dL × 12.87 = 23.2 pmol/L – the upper end of a typical adult range.
• 12 pmol/L ÷ 12.87 = 0.93 ng/dL.
• 22 pmol/L ÷ 12.87 = 1.71 ng/dL.
• 40 pmol/L ÷ 12.87 = 3.11 ng/dL – well above the reference range, as seen in overt hyperthyroidism.

Mental shortcut

Multiply ng/dL by 13 for an estimate slightly on the high side, or divide pmol/L by 13. For 1.3 ng/dL: 1.3 × 13 = 16.9 pmol/L (exact: 16.7).

Free T4 conversion chart

Normal free T4 ranges

A typical adult reference range is about 0.8–1.8 ng/dL (10–23 pmol/L). Laboratory ranges vary substantially because free T4 immunoassays are not fully standardised; ranges such as 0.9–1.7 ng/dL or 12–22 pmol/L are common. Always use the range on your own report.

Children

Newborns have very high free T4 for the first few days of life, after a surge of TSH at birth. Levels then fall gradually through infancy and childhood. Paediatric reference ranges are age-specific, and newborn screening programmes test TSH (and in some countries T4) to detect congenital hypothyroidism.

Older adults

Free T4 ranges change little with age, but TSH tends to rise in older people. Some guidelines therefore accept a higher TSH in people over about 70–80 before treating mild hypothyroidism.

Reading free T4 together with TSH

Free T4 is almost always interpreted alongside TSH. The common patterns are:

The last two patterns are uncommon and usually need specialist review and repeat testing with a different assay.

Subclinical thyroid disease

Subclinical hypothyroidism means a raised TSH with a normal free T4. Treatment decisions depend on the TSH level (many guidelines suggest treatment when TSH is persistently above 10 mIU/L), symptoms, age, pregnancy plans and thyroid antibodies. Subclinical hyperthyroidism – low TSH with normal free T4 and T3 – may be treated when TSH is fully suppressed, especially in older adults, because it increases the risk of atrial fibrillation and bone loss.

Free T4 during levothyroxine treatment

Levothyroxine is synthetic T4. In people taking it for hypothyroidism, treatment is adjusted mainly by TSH, aiming for a TSH within the reference range in most adults. Free T4 often sits in the upper half of the reference range, or slightly above it, on a dose that normalises TSH, because the body relies more on converting T4 to T3 when the thyroid gland itself is not working.

Timing matters: free T4 rises for several hours after a levothyroxine dose. If you take your tablet in the morning and have blood drawn shortly afterwards, free T4 may be falsely high. Some guidelines suggest taking the dose after the blood test on testing days. TSH is little affected by the timing of the dose.

After a dose change, wait about six to eight weeks before rechecking TSH and free T4, because T4 has a half-life of about a week and takes several weeks to reach a new steady state.

Free T4 in pregnancy

Pregnancy changes thyroid tests in several ways. High levels of hCG in the first trimester stimulate the thyroid, lowering TSH and slightly raising free T4. Rising oestrogen increases TBG, raising total T4. Later in pregnancy, many free T4 immunoassays read lower than the true value.

For these reasons, the American Thyroid Association recommends using trimester-specific reference ranges, ideally derived locally, and accepting that a lower free T4 in later pregnancy may reflect the assay rather than true hypothyroidism. Women already taking levothyroxine usually need a dose increase of about 20–30% as soon as pregnancy is confirmed, with TSH checked every four weeks in the first half of pregnancy.

Thyroid antibodies and the cause of abnormal free T4

When free T4 and TSH show thyroid dysfunction, antibody tests help identify the cause:

• Thyroid peroxidase antibodies (TPOAb) are positive in most people with Hashimoto's thyroiditis, the commonest cause of hypothyroidism in iodine-sufficient countries. In people with subclinical hypothyroidism, positive TPOAb predict a higher chance of progressing to overt hypothyroidism – roughly 4% per year compared with about 2% without antibodies.
• TSH receptor antibodies (TRAb) are found in Graves' disease, the commonest cause of hyperthyroidism. A positive TRAb confirms Graves' disease and helps distinguish it from thyroiditis or toxic nodules.

Thyroiditis – for example after a viral illness, after pregnancy (postpartum thyroiditis) or with some medicines such as immune checkpoint inhibitors – often causes a temporary high free T4 followed by a low phase, before recovery. Repeating free T4 and TSH every four to six weeks shows which phase a person is in.

Hyperthyroidism and free T4 levels

In overt hyperthyroidism, free T4 may be only slightly above the range or several times higher. As a rough guide, values of 2–3 times the upper limit of normal (for example 45–70 pmol/L, 3.5–5.4 ng/dL) are common in untreated Graves' disease. The free T4 level helps guide the starting dose of antithyroid medicines such as carbimazole or methimazole, and it is monitored every four to six weeks during early treatment. Because TSH can remain suppressed for months after free T4 normalises, free T4 (and free T3) rather than TSH guides dose adjustment in the early phase of treatment.

Comparing results from different laboratories

Free T4 assays from different manufacturers can give results that differ by 10–20% for the same sample, and reference ranges differ to match. This is particularly noticeable for people who change laboratory or move country. A practical approach is to convert values into one unit, compare each result with its own laboratory's range (for example, as a position within the range), and give more weight to TSH, which is better standardised. For example, 1.25 ng/dL at a US laboratory with a range of 0.82–1.77 ng/dL and 17 pmol/L at a UK laboratory with a range of 12–22 pmol/L are both close to the middle of their ranges, even though 1.25 ng/dL converts to 16.1 pmol/L.

What affects free T4 results

• Levothyroxine timing – samples taken within a few hours of a dose read higher.
• Biotin – high-dose biotin (common in hair, skin and nail supplements) interferes with many immunoassays and can produce falsely high free T4 and falsely low TSH, mimicking hyperthyroidism. Stop biotin for at least two days, and longer for very high doses, before testing.
• Heparin – even small doses can release free fatty acids in the sample tube and falsely raise free T4.
• Medicines – amiodarone, high-dose glucocorticoids, dopamine and some anticonvulsants change thyroid hormone production, conversion or binding. Phenytoin and carbamazepine can lower measured free T4 on some assays.
• Serious illness – "non-thyroidal illness" can lower T3, sometimes T4, and briefly change TSH, without true thyroid disease. Thyroid tests are best delayed until recovery unless thyroid disease is suspected.
• Binding protein abnormalities – rare inherited variants (such as familial dysalbuminaemic hyperthyroxinaemia) can make some assays read falsely high.
• Heterophile or anti-T4 antibodies – can interfere with immunoassays and produce inconsistent results.

Iodine, supplements and thyroid tests

The thyroid needs iodine to make T4 and T3. Iodine deficiency remains a cause of goitre and hypothyroidism worldwide, but in countries with iodised salt it is uncommon. Too much iodine can also disturb thyroid function: high-dose kelp or seaweed supplements, iodine-containing "thyroid support" products, amiodarone and iodinated contrast for CT scans can trigger either hypothyroidism or hyperthyroidism in susceptible people. Some over-the-counter "thyroid glandular" supplements contain actual thyroid hormone and can raise free T4 and suppress TSH. Tell your clinician about any supplements before thyroid testing.

When to see a doctor

See a doctor if your free T4 or TSH is outside the reference range, or if you have symptoms of an underactive thyroid (tiredness, feeling cold, weight gain, constipation, dry skin, heavy periods, low mood) or an overactive thyroid (weight loss, heat intolerance, palpitations, tremor, anxiety, diarrhoea, eye changes). Seek urgent care if you have a very fast or irregular heartbeat, fever and confusion with known hyperthyroidism, or severe drowsiness, low temperature and slowed breathing with known hypothyroidism – rare but serious emergencies. Pregnant women with known thyroid disease should contact their clinician as soon as pregnancy is confirmed.