To evaluate the role of serum T4- and T3-binding proteins in the elevation of serum T4 and T3 concentrations in the woodchuck in the fall and winter, blood was collected from woodchucks during the four seasons of the year (seasonal study) and from 2-week fasted woodchucks in the summer (fasting study) and the serum concentrations of total T4 and T3 were measured. The distribution of [125I]4 and [125I]T3 tracers among the serum binding proteins and the serum T4-binding globulin (TBG) binding capacity for T4 was determined by polyacrylamide gel electrophoresis. Plasma concentrations of both T4 and T3 were highest in the winter. The major T4-binding protein in the woodchuck is TBG. There was an increase in both [125I]T4 and [125I]T3 tracer binding to serum TBG in fall and winter, and TBG binding capacity for T4 was 2-fold higher in winter than in summer. There were increases in TBG binding and in the TBG T4 binding capacity in the 2-week starved animals. The increased binding of T4 and T3 by TBG in the fall and winter may be partially responsible for the increased serum concentrations of T4 and T3 in the fall and winter in the woodchuck, a time when secretion of T4 and T3 by the thyroid gland is very low. This may be facilitated by the low or absent food consumption at these times of the year.