Your TSH came back high (or low) and your doctor ordered Free T4 next. That sequence is not random. TSH alone tells you what the brain thinks. Free T4 tells you what the thyroid is actually delivering. Read together, the pattern almost always names the disease.
The "free" part matters. About 99.97% of the T4 in your blood is bound to carrier proteins and metabolically silent. Only the tiny unbound fraction can enter cells and get converted to active T3. Total T4 reacts to anything that shifts those carriers (pregnancy, estrogen, the pill, nephrotic syndrome). Free T4 cuts through that noise.
What Free T4 measures
Your thyroid mostly makes T4. T3, the more active hormone, is largely produced downstream when tissues convert T4 locally. That's why Free T4 is the steadier read on hormone supply, while Free T3 jumps around.
- Total T4: outdated for most uses. Dragged around by binding-protein changes.
- Free T4 (direct or by index): the standard test today.
- Free T4 by equilibrium dialysis: gold standard, reserved for cases where the standard assay is unreliable (severe illness, antibody interference).
Free T4 reference range
| Grupo demográfico | Bajo | Alto | Unidad |
|---|---|---|---|
| Adults — standard range | 0.8 | 1.8 | ng/dL |
| Adults — SI units | 10 | 23 | pmol/L |
| Pregnancy — first trimester | 0.8 | 1.7 | ng/dL |
| Pregnancy — second/third trimester | 0.6 | 1.4 | ng/dL |
| Children 1–18 years | 0.9 | 1.7 | ng/dL |
| Newborns (first week) | 2 | 5 | ng/dL |
| Overt hypothyroidism | 0 | 0.7 | ng/dL |
| Overt hyperthyroidism | 1.9 | 8 | ng/dL |
Most labs report 0.8–1.8 ng/dL for adults, though exact cutoffs depend on the assay. SI units run roughly 10–23 pmol/L (multiply ng/dL by ~12.9).
- 0.8–1.8 ng/dL: standard adult range.
- Below 0.8 ng/dL: low. Primary hypothyroidism if TSH is high. Central hypothyroidism if TSH is low or inappropriately normal.
- Above 1.8 ng/dL: high. Primary hyperthyroidism if TSH is suppressed. Rare TSH-secreting pituitary tumor if TSH is normal or high.
- Pregnancy: Free T4 drifts down across trimesters as binding proteins climb. Use trimester-specific ranges when available.
- Newborns: much higher than adults in the first weeks.
A "normal" Free T4 with an obviously abnormal TSH still tells you something. It usually means subclinical disease, not overt.
What high Free T4 means
Too much circulating thyroid hormone. The TSH pairing narrows the cause:
- High Free T4 + low TSH: overt primary hyperthyroidism. Usually Graves' disease, a toxic multinodular goiter, or a toxic adenoma.
- High Free T4 + high or normal TSH: rare. TSH-secreting pituitary adenoma or thyroid hormone resistance. Endocrinology, please.
- High Free T4 + suppressed TSH after a recent levothyroxine dose: over-replacement. Drop the dose, recheck in 6–8 weeks.
- Thyroiditis (early phase): viral, postpartum, or silent thyroiditis dumps stored hormone. Transient hyperthyroidism that resolves in weeks.
- Iodine load: amiodarone or iodinated contrast can tip a susceptible thyroid into hyperthyroidism.
- Factitious thyrotoxicosis: surreptitious hormone use. Thyroglobulin is low, which separates it from endogenous causes.
Symptoms: palpitations, weight loss, heat intolerance, anxiety, tremor, insomnia, frequent stools, menstrual changes. Older adults often present quietly: isolated atrial fibrillation, weight loss, or apathy. Easy to miss.
What low Free T4 means
Not enough hormone reaching tissues. Pair it with TSH:
- Low Free T4 + high TSH: overt primary hypothyroidism. Hashimoto's leads the field. Others: post-surgical, post-radioiodine, severe iodine deficiency, drugs (lithium, amiodarone, immune checkpoint inhibitors).
- Low Free T4 + low or inappropriately normal TSH: central hypothyroidism. Pituitary tumors, surgery, radiation, infiltrative disease, traumatic brain injury. Image the pituitary; run a full anterior pituitary panel.
- Low Free T4 + low TSH in critical illness: non-thyroidal illness syndrome ("euthyroid sick"). Treat the underlying illness. Don't reflexively start levothyroxine.
- Low Free T4 in pregnancy with normal TSH: isolated hypothyroxinemia. Management is debated. Some specialists treat in the first trimester to support fetal brain development.
Hypothyroid symptoms are vague: fatigue, cold intolerance, weight gain, dry skin, hair loss, constipation, slow thinking, low mood, heavy menses. Severe untreated cases progress to myxedema coma, which is a true emergency.
Reading Free T4 in context
Free T4 is steadier than Free T3, but it still gets pushed around by:
- Recent levothyroxine dose: peaks 2–4 hours after an oral dose. Draw before the daily dose.
- Biotin supplements: high-dose biotin scrambles many immunoassays and can falsely raise Free T4. Stop for at least 48 hours before the test.
- Severe illness: non-thyroidal illness suppresses Free T4 transiently. Skip routine thyroid testing during acute hospitalization unless thyroid disease is strongly suspected.
- Heparin: both unfractionated and low-molecular-weight forms can falsely elevate Free T4 by displacing it from binding proteins.
- Antibody interference: anti-T4 and heterophile antibodies cause unpredictable assay results. Equilibrium dialysis is the workaround.
For monitoring levothyroxine, TSH is the primary target and Free T4 is supportive. After any dose change, wait 6–8 weeks before retesting so TSH equilibrates.
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When to act on Free T4
- High Free T4 with suppressed TSH: confirm overt hyperthyroidism. Workup (TSH receptor antibodies, radioiodine uptake or thyroid ultrasound), beta-blocker for symptoms.
- Low Free T4 with high TSH: confirm overt hypothyroidism. Start levothyroxine, check anti-TPO antibodies for Hashimoto's.
- Low Free T4 with low or normal TSH: central hypothyroidism until proven otherwise. Endocrinology referral, pituitary imaging.
- Discordant pattern (high Free T4 + high TSH): rare. Refer for TSH-secreting adenoma or thyroid hormone resistance evaluation.
- Pregnancy with abnormal Free T4: refer to maternal-fetal medicine or endocrinology. Both under- and over-treatment affect fetal development.
- New atrial fibrillation with high-normal or high Free T4: even subclinical hyperthyroidism raises AF and stroke risk in older adults.
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Tests that complete the picture
- TSH: always read Free T4 with TSH. The pattern names the disease.
- Free T3: useful when hyperthyroidism is suspected (T3 toxicosis) and on combination T4/T3 therapy.
- Anti-TPO antibodies: confirm Hashimoto's when Free T4 is low and TSH is high.
- TSH receptor antibodies (TRAb): confirm Graves' when Free T4 is high and TSH is suppressed.
- Thyroglobulin: separates endogenous hyperthyroidism from factitious thyrotoxicosis (low in factitious cases).
- Pituitary panel (cortisol, ACTH, prolactin, LH/FSH, IGF-1): when central hypothyroidism is on the table.