fT3 / fT4 ratio
Stomach and gut healthAlso known as: free T3 to free T4 ratio, T3/T4 ratio, Schilddrüsenhormon-Verhältnis
The ratio of free triiodothyronine (fT3) to free thyroxine (fT4) reflects how efficiently your body converts the storage thyroid hormone (T4) into the active form (T3).
A low ratio may indicate impaired conversion, which can occur with stress, illness, selenium deficiency, or certain medications — even when individual thyroid values appear normal.
Why this matters
The ratio helps identify conversion problems that standard thyroid tests might miss. A high ratio may indicate hyperthyroidism or excessive T4-to-T3 conversion, while a low ratio suggests poor conversion often seen in chronic illness, stress, nutritional deficiencies (selenium, zinc), or certain medications. This is particularly valuable for patients with normal TSH but persistent hypothyroid symptoms, as it can reveal "functional hypothyroidism" where adequate hormone is produced but not properly activated.
How this connects to other biomarkers
- fT3/fT4 ratio reflects how effectively the body converts T4 (storage form) into the active T3 hormone. Low ratios suggest impaired conversion (chronic illness, selenium/iron deficiency, chronic stress, abnormal Cortisol) and may correlate with elevated Reverse T3.
- High ratios suggest preferential T3 production, often seen in iodine deficiency or autonomous (overactive) thyroid nodules.
- The ratio is most informative when TSH, fT3, and fT4 don't fit a clean classical pattern.
How often should I test fT3 / fT4 ratio?
The fT3/fT4 ratio is reassessed whenever both free T3 and free T4 are measured, alongside the full thyroid panel rather than on its own schedule.
At baseline / for screening: Once every 12 months from age 30. More frequently if you have a family history of thyroid disease, are pregnant or planning pregnancy, have an autoimmune condition, or notice symptoms suggesting thyroid dysfunction (energy, weight, mood, temperature regulation).
When monitoring an intervention or change: Retest 6 to 8 weeks after starting or adjusting thyroid medication (levothyroxine, liothyronine, antithyroid drugs), since that's how long TSH takes to reach a new steady state. Acute illness and severe caloric restriction can shift free T3 downward independent of thyroid function, so wait for those to resolve before reading the trend.
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