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01Markers · Thyroid

Free T4

Free thyroxine

T4 is the main hormone the thyroid releases; the body converts it to T3. The free T4 test measures the share not bound to proteins.

Measured in
Blood (serum)
Units
ng/dL, pmol/L
Also called
FT4, thyroxine, T4
  1. Ashwagandha
  2. Iron
  3. Magnesium
  4. Berberine
  5. Collagen
  6. Creatine
  7. Finnish sauna
  8. HIIT
  9. L-carnitine
  10. L-theanine
  11. Lion's mane
  12. Melatonin
  13. Milk thistle
  14. NAC
  15. NAD+
  16. Omega-3
  17. PQQ
  18. Red yeast rice
  19. Resveratrol
  20. Taurine
  21. Ubiquinol (CoQ10)
  22. Vitamin C
  23. Vitamin D3
  24. Vitamin E
  25. Vitamin K2
  26. Zinc
  27. Zone 2 cardio
Free T4 across the 27 supplements and practices researched so far · best evidence first

AWhat moves Free T4

3 of 27 studied

In human studies, 2 of the 27 supplements and practices researched so far raised free T4. 1 showed no change. 24 were never studied for it.
  • Supported 2
  • No effect 1
  • Conflicting 0
  • Not studied 24
See Free T4 on the evidence map →Check claims about Free T4 →

Moves it2

Human studies found a change. The grade reflects the studies behind it, from A (strongest) to D.

AshwagandhaRaisesGrade C

Ashwagandha raises free T4.

Lab or animal studies · 1 study linked

Studied in: Subclinical hypothyroid patients, Not studied in euthyroid

Open the Ashwagandha page →Check a claim about this →
IronRaisesGrade C

Iron raises free T4.

Small effect · 0 studies linked

This result is graded, but no study is linked to it yet. Treat it as unconfirmed.

Studied in: Iron deficient

Similar mechanism to T3 via thyroid peroxidase. Modest increase with iron repletion in iron-deficient populations.

Open the Iron page →Check a claim about this →

Studied, no change1

Studies looked and found no meaningful change. That is a result, not missing data.

MagnesiumNo changeGrade D

Studies of Magnesium found no change in free T4.

0 studies linked

This result is graded, but no study is linked to it yet. Treat it as unconfirmed.

Studied in: Adults

Grade D pending dedicated thyroid-focused Mg RCTs.

Open the Magnesium page →Check a claim about this →

Not studied24

No usable human study has measured Free T4 with these. A gap is not a negative result: nobody has looked.

BerberineCollagenCreatineFinnish saunaHIITL-carnitineL-theanineLion's maneMelatoninMilk thistleNACNAD+Omega-3PQQRed yeast riceResveratrolTaurineUbiquinol (CoQ10)Vitamin CVitamin D3Vitamin EVitamin K2ZincZone 2 cardio

Commission research on a gap →

Why trials measure it. Reported with TSH to describe thyroid function in trials.

About this marker: Thyroxine (T4) Test (MedlinePlus, U.S. National Library of Medicine).

Evidence states come from published human studies found in PubMed and OpenAlex and read by a model pipeline. "Not studied" means no usable human study was found for this marker, which is a gap, not a negative result. Every result links its studies so you can check it.

Research summary for education. Not medical advice.