FT3/FT4 molar ratio

A crude proxy for peripheral T4-to-T3 conversion (deiodinase activity).
Also known as:
FT3-FT4-ratio
.
Do not confuse with FT3/FT4 mass ratio, which results in different values.
Enter values
 
Value
Unit
fT3
fT4

FT3/FT4 molar ratio ranges

According to OptimalDX, FT3/FT4 molar ratio should be between 0.29 and 0.32:
0.29
0.32
Healthy thyroid function was associated with FT3/FT4 molar ratio between 0.27 and 0.46:
0.27
0.46
Healthy thyroid function was associated with FT3/FT4 molar ratio between 0.27 and 0.39:
0.27
0.39
Increased all-cause mortality was associated with FT3/FT4 molar ratio below 0.44:
0.44
0.48
Risk Scale
Optimal range for FT3/FT4 molar ratio appears to be between 0.29 and 0.38:
0.1
0.27
0.29
0.38
0.42
0.6

A crude proxy for peripheral T4-to-T3 conversion (deiodinase activity).

FT3/FT4 molar ratio interpretation

For low

A low FT3/FT4 ratio indicates relatively low FT3 compared with FT4.

  • Common in patients on T4-only monotherapy (Levothyroxine).
  • With normal FT4 and TSH: poor peripheral conversion. Seen in: Insufficient Selenium or Zinc, Chronic inflammation, high cortisol/stress, or caloric restriction.
  • Decreased ratio seen in unhealthy aging (often with high BMI).
  • Very low ratio seen in non-thyroidal illness (acute or chronic systemic disease).
Sources

For high

A high FT3/FT4 ratio indicates relatively high FT3 compared with FT4.

  • Common in patients taking T3-containing medication.
  • With low FT4 and/or high TSH: untreated early Hashimoto’s hypothyroidism. Compensatory mechanism via upregulating D2 enzyme (enhancing peripheral T4-T3 conversion rate) and TSH receptor stimulation (increasing the T3/T4 ratio produced by the thyroid gland).
  • With elevated FT4 and TSH < 0.01: untreated early Graves’ hyperthyroidism. TSH-receptor stimulating antibodies are hijacking TSH receptors and disproportionately increasing T3 production (often differentiates Graves' from thyroiditis).
  • With low or "inappropriately normal" TSH: Central hypothyroidism.
  • Elevated FT3/FT4 ratio is also seen in some euthyroid individuals with obesity, insulin resistance, or metabolic syndrome.
Sources

Images:

FT3/FT4 decrease during aging

FT3/FT4 decrease during aging

Chinese euthyroid with sufficient iodine intake
Low FT3/FT4 increase all-cause mortality risk

Low FT3/FT4 increase all-cause mortality risk

Euthyroid US adults
Dataset: NHANES 2007-2012

About the site author

My personal mission is to extend human lifespan by collectively adding 1 million years to people's lives.

That’s why I made this site 100% free—no sign-up, no email. Just instant results.

Zsolt Szabo
Zsolt Szabo
Precision Health Consultant for Longevity Clinics, Biohacker, and Software Engineer

How would you improve this tool ?

Any feedback is welcome at .