Figure 1.
Free thyroid hormones and FT3/FT4 ratio frequency distribution.
FT3 and FT4 serum levels and FT3/FT4 ratio distribution in 1,811 athyreotic patients under levothyroxine (L-T4) monotherapy. Shaded areas indicate the normal range (2.5–97.5 percentiles) calculated in 3,875 euthyroid controls. Vertical dotted lines indicate the median of the normal values. Percentages indicate the patients with values under or above the normal values.
Table 1.
TSH, FT4, FT3 levels in studied subjects by age and gender.
Figure 2.
Correlation between TSH and free thyroid hormones in euthyroid controls and in athyreotic patients.
The correlation between TSH serum levels (log values) and FT3 and FT4 serum levels in 3,875 euthyroid controls (solid lines) and 1,811 athyreotic patients under levothyroxine monotherapy (dotted lines). The linear regression equations between FT4 and log TSH levels in the euthyroid controls and the levothyroxine (L-T4)-treated patients were y = 14.0−1.1x (95%: slope −1.4 to −0.74) and y = 16.1−2.01x (95% CI: slope −2.48 to −1.53), respectively. The same curve fitting analysis was used between FT3 and log-TSH levels and resulted in the following: y = 4.4−0.029x (95% CI: slope −0.128 to 0.063) for euthyroid controls and y = 3.84−0.575x (95% CI: slope −0.697 to −0.453) for L-T4-treated patients. R square and P values are reported in the graph.
Figure 3.
FT3/FT4 ratio in levothyroxine-treated athyreotic patients at different daily dose.
Median FT3/FT4 ratio in levothyroxine (L-T4) treated athyreotic patients with respect to the administered daily dose of L-T4. The median and interquartile range in the euthyroid controls are indicated by the shaded area. The number of patients with a FT3/FT4 ratio lower than the 2.5 percentile of the euthyroid controls is indicated in the right panel.
Table 2.
TSH, FT4, FT3 levels in euthyroid controls and in levothyroxine-treated athyreotic patients subdivided according to TSH levels.