fT3/rT3 ratio
Free T3 to reverse T3 ratio reflects the balance between thyroid hormone activation (T4→T3) and inactivation (T4→rT3).
There are multiple competing formulas, this uses the most common one: (fT3 in pg/mL ÷ rT3 in ng/dL) × 100. Some formulas also use total T3 instead of free T3.fT3/rT3 ratio interpretation
For low
Shift toward increased thyroid hormone inactivation (higher rT3, lower T3) is driven by systemic illness, inflammation, energy deficit, or organ dysfunction rather than an "thyroid problem".
For high
Shift toward decreased thyroid hormone inactivation (lower rT3, higher T3).
- With low PTFQI it is supportive of central hypothyroidism.
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