Seasonal Variation and the Influence of Body Temperature on Plasma Concentrations and Binding of Thyroxine and Triiodothyronine in the Woodchuck*

Abstract

Woodchuck plasma was collected during four seasons of the year and assayed for total and dialyzable (free) T₄ and T₃ and for rT₃. Plasma concentrations of total and free T₄ and T₃ were higher in the spring (T₄, 5.4 ± 0.6 μg/dl; free T₄, 3.0 ± 0.4 ng/dl; T₃) 202 ± 22 ng/dl; free T₃) 0.51 ± 0.04 ng/dl) and lower in the prehibernatory fattening period in summer (T₄) 2.3 ± 1.0 μg/dl; free T₄, 1.2 ± 0.5 ng/dl; T₃, 45 ± 27 ng/dl; free T₃, 0.16 ± 0.10 ng/dl) and fall (T₄, 3.2 ± 1.0 μg/dl; free T₄) 1.3 ± 0.2 ng/dl; T₃, 130 ± 12 ng/dl; free T₃, 0.25 ± 0.02 ng/dl). In spite of the extremely high concentrations of T₃ in the winter (437 ± 32 ng/dl), free T₃ concentrations (0.034 ± 0.003 ng/dl), when measured at the appropriate temperature for hibernation, were significantly lower than those found at other seasons of the year. Plasma binding of T₃ was lower during the summer and increased again to approximately double the spring value during the winter. rT₃ was at or below the sensitivity of the method (6 ng/dl) at all seasons. It is suggested that the wide seasonal variations in thyroid hormone concentrations and altered plasma protein binding may represent important adaptations influencing the metabolic rate and the process of hibernation in the woodchuck.