Circulating Levels of Thyroxine (T₄) and Triiodothyronine (T₃) in the Migratory Canada Goose

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Abstract
Changes in circulating levels of thyroxine (T₄) and triiodothyronine (T₃) in the migratory Canada goose (Branta canadensis interior) during the different periods of its yearly life cycle in both its breeding and wintering grounds were investigated. Total plasma T₄ level ranged from a minimal value of 1.156 μg/dl in the fall postmigratory period to a maximal value of 1.737 μg/dl in the spring postmigratory period. The T₄ values during spring postmigratory period and breeding (1.700 μg/dl) were significantly higher than those in the spring premigratory (1.175 μg/dl), fall premigratory (1.327 μg/dl), and the fall postmigratory periods. The T₄ level during molt was significantly higher than that during the spring premigratory period and the fall postmigratory period but was not significantly different from that in the other periods. The peak level in plasma T₃ (330.62 ng/dl) observed in the spring premigratory phase was significantly higher than that in all other periods, and the trough T₃ level (71.72 ng/dl) noted in the fall premigratory period was significantly lower than that in all the other periods. No significant variations in T₃ levels were noted during the spring postmigratory, breeding, molt, and the fall postmigratory periods. The T₃ level was at its peak in the spring premigratory period when the ambient temperature was 20-25 C and daily photoperiod 11.5 h. However, the T₄ levels were highest in the spring postmigratory and breeding periods when the ambient temperatures were from -12 to -3 C and from -3 to 0 C and the photoperiods 16 h and 18 h, respectively. It is concluded that the high T₄ levels during these periods are indicative of the influence of the lower ambient temperatures and greater day lengths on the activity of the thyroid and the activity of the animal. The hypertrophy of the pectoral muscles in the spring pre- and postmigratory periods may be attributed to increased thyroidal activity. No definite relationship of thyroidal activity to molt was discernible, however.