Seasonal changes in thyroid activity in the lesser snow goose (Anser caerulescens caerulescens) including reference to embryonic thyroid activity

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Abstract
Plasma thyroxine (T4) levels in adult lesser snow geese (Anser caerulescens caerulescens) increased concomitantly with gonadal recrudescence and remained elevated until shortly before moult. This pattern of declining plasma T4 concentrations preceding moult was evident in gosling, yearling, and adult birds. Plasma triiodothyronine (T3) concentrations did not change significantly throughout the year. There was no apparent correlation between plasma thyroid hormone levels and thyroid weight or thyroid epithelial cell height. Plasma levels of both T3 and T4 increased in goslings shortly before hatch and on day 20 of incubation reached levels (0.58 ± 0.36 and 19.5 ± 2.5 ng∙mL−1, respectively) similar to those found in the plasma of newly hatched birds (0.76 ± 0.24 and 36.0 ± 6.5, respectively).