fT4 (free thyroxine)
fT4 (free thyroxine) is the main thyroid hormone in its active, free form. The thyroid secretes it on the command of TSH; in tissues it is converted to the even more active fT3. It reflects the actual availability of thyroid hormones.
| Rating | Range | What it means |
|---|---|---|
| Talk to a doctor | below 12 pmol/L | Below the reference — seek advice. |
| Good | 12 – 22 pmol/L | At optimum. |
| Talk to a doctor | 22 pmol/L and above | Above the reference — seek advice. |
Why it matters
ETA: together with TSH it shows the actual thyroid function.
If it’s low
Low fT4 (with high TSH) means too little hormone (hypothyroidism) — fatigue, feeling cold, weight gain, dry skin, constipation.
If it’s high
High fT4 means too much thyroid hormone (hyperthyroidism) — palpitations, tremor, weight loss, anxiety, sweating.
What to do
Interpret fT4 with TSH (and antibodies if needed). Treatment (replacement or suppression) is a medical matter; lifestyle is only supportive.
Read together with
TSH · fT3 (free triiodothyronine) · Anti-TPO antibodies
Part of the guide: Thyroid →
Articles about this marker
- Hashimoto's: what it is, what is proven to help and what the latest research saysMarker explained
- Thyroid: which tests tell you what, and how to do them properlyMarker explained
- TSH: normal values, what a high TSH means and when it is a diseaseThyroid
This content is for information only and is not a substitute for medical advice. Talk to your doctor about your results.
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