High FT3:FT4 Ratio

A high ratio suggests that FT3 is elevated relative to FT4.
In Hyperthyroidism, T3 may rise more markedly than T4, producing a pattern sometimes called “T3toxicosis.”
Therefore, a high FT3:FT4 ratio can indicate more active or T3dominant hyperthyroidism, helping differentiate cases where excess T3 production is driving symptoms.

Low FT3:FT4 Ratio

A low ratio suggests relatively lower FT3 compared with FT4.
T3 measurement is not routinely assessed and T4 and TSH are primary markers for hypothyroidism.
A low ratio may therefore reflect reduced peripheral conversion of T4 to T3, which can occur in nonthyroidal illness or hypothyroidism, but must be interpreted with TSH and clinical findings.