Understanding Free T4 (sT4) and Thyroid Function
When a patient walks into my clinic feeling tired, anxious, or inexplicably gaining weight, the thyroid is often the first place I look. The thyroid gland produces two main hormones: thyroxine (T4) and triiodothyronine (T3). Free T4 (sT4 in Turkish labs) is the active, unbound fraction of T4 that can enter cells and regulate your metabolism. Unlike total T4, which includes bound hormone, FT4 gives a true picture of what your body can actually use.
Why Doctors Order the Free T4 Test
In endocrine practice, we rarely check FT4 alone. It is almost always paired with TSH (thyroid-stimulating hormone). TSH tells us how the pituitary is responding, while FT4 reveals the actual hormone output. I typically order FT4 when I suspect:
- Hyperthyroidism (overactive thyroid)
- Hypothyroidism (underactive thyroid)
- Monitoring thyroid hormone replacement therapy
- Pituitary or hypothalamic disorders
- Evaluating thyroid nodules or goitre
Normal Reference Ranges for Free T4
Reference ranges can vary by laboratory and assay method. The following table shows typical values in pmol/L and ng/dL. Always interpret with your lab's specific range.
| Age / Population | Free T4 (pmol/L) | Free T4 (ng/dL) |
|---|---|---|
| Adults (18–65 years) | 9–22 | 0.7–1.7 |
| Older adults (>65 years) | 9–19 | 0.7–1.5 |
| Children (1–12 years) | 10–23 | 0.8–1.8 |
| Pregnant women (1st trimester) | 7–18 | 0.5–1.4 |
| Pregnant women (2nd trimester) | 6–16 | 0.5–1.2 |
| Pregnant women (3rd trimester) | 5–14 | 0.4–1.1 |
What Does a Low Free T4 (sT4) Mean?
Low FT4 with a high TSH often points to primary hypothyroidism – the thyroid itself isn't producing enough hormone. Common causes include autoimmune Hashimoto's thyroiditis, iodine deficiency, or previous thyroid surgery. Symptoms I see in clinic include fatigue, cold intolerance, dry skin, constipation, and brain fog.
What Does a High Free T4 Mean?
Elevated FT4 with suppressed TSH suggests hyperthyroidism, often from Graves' disease, a toxic nodule, or thyroiditis. My patients with high FT4 describe palpitations, heat intolerance, weight loss despite good appetite, anxiety, and tremors. It's essential to differentiate primary hyperthyroidism from secondary causes like a TSH-secreting pituitary tumour (much rarer).
Free T4 During Pregnancy
Pregnancy dramatically affects thyroid function. Human chorionic gonadotropin (hCG) can mildly stimulate the thyroid, lowering TSH and sometimes raising FT4 slightly. However, FT4 normally decreases as pregnancy advances due to increased binding proteins. We use trimester-specific reference ranges to avoid misdiagnosing thyroid dysfunction. Untreated hypothyroidism in pregnancy carries risks for both mother and baby, so monitoring FT4 along with TSH is standard.
Medications That Affect Free T4
Several drugs can alter FT4 levels:
- Levothyroxine: Replacement therapy, expected to normalise FT4.
- Antithyroid drugs (methimazole, propylthiouracil) lower FT4.
- Amiodarone can cause both hypo- and hyperthyroidism by damaging thyroid tissue.
- Oestrogen (oral contraceptives, HRT) increases TBG, which can lower free T4 slightly but usually stays in range.
- Glucocorticoids, dopamine, and somatostatin analogues can suppress TSH and lower FT4.
Always tell your doctor about all medications and supplements before testing.
Lifestyle Factors and Free T4
In my practice, I remind patients that severe illness, fasting, or major stress can temporarily lower FT4 (euthyroid sick syndrome). This is a protective response and usually doesn't require treatment. Iodine intake matters: both deficiency and excess can derange FT4 levels. If you're on biotin supplements, they can interfere with some lab assays, so stop biotin 72 hours before a thyroid test.
How Is the Free T4 Test Performed?
It's a simple blood draw from a vein, usually in the arm. No special preparation is needed, though morning sampling is recommended because TSH has a diurnal rhythm. The sample is sent to the lab, where either a chemiluminescent immunoassay (CLIA) or liquid chromatography-tandem mass spectrometry (LC-MS/MS) measures FT4. Results typically come back within 24 hours.
When Should You Repeat the Test?
If you have a borderline abnormal result, your doctor may repeat the test in 4–6 weeks, along with TSH and sometimes TPO antibodies. For those on thyroid medication, FT4 is checked 6–8 weeks after any dose change. Annual monitoring is standard once stable.
Frequently Asked Questions
Frequently Asked Questions
What is the difference between Free T4 and Total T4?
Total T4 measures both bound and unbound thyroxine in the blood. Free T4 (sT4) measures only the small fraction not attached to proteins. Since only free T4 can enter cells, FT4 better reflects thyroid function. Total T4 can be misleading in pregnancy, oral contraceptive use, or liver disease because protein levels change.
Can stress cause abnormal Free T4 levels?
Yes, severe physical or emotional stress can temporarily lower Free T4 due to the euthyroid sick syndrome. This is a non-thyroidal illness adaptation. In chronic stress, subtle changes in TSH and FT4 may occur but usually return to baseline once the stressor resolves. Routine thyroid testing is best done when you are well.
Is high Free T4 always dangerous?
Not always, but persistently high Free T4 often indicates hyperthyroidism, which can lead to serious complications like atrial fibrillation, osteoporosis, and thyroid storm. However, mild elevation with a normal TSH may be due to assay interference, pregnancy, or certain drugs. Always correlate with symptoms and repeat testing.
About Free T4 (FT4)
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Scientific Sources & References
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