Figure 2.

Relationship between TSH and FT4 or FT3, stratified by quartiles of deiodinase activity (a, c), administered LT4 dose (b) or FT3 concentrations (d).

The main panel shows the crude relationship obtained by locally weighted scatterplot smoothing in each strata over the full range. The inserts are restricted to the nonhypothyroid range, fitted by a mixed model using natural splines for the independent variable and its interaction with the respective strata after adjusting for gender, age and body mass index (except if dose was already weight-adjusted). The curves shown are significantly different (see Results).

FT3, free triiodothyronine; FT4, free thyroxine; LT4, levothyroxine; TSH, thyroid stimulating hormone.