Changes in plasma thyroid hormone levels in pink salmon, Oncorhynchus gorbuscha, during their spawning migration in the Fraser River (Canada)

Plasma thyroid hormone levels were measured in pink salmon, Oncorhynchus gorbuscha, during their spawning migration in the Fraser River, British Columbia. The plasma levels of both l-thyroxine (T4) and triiodo-l-thyronine (T3) were significantly higher in males than in females. In both sexes the hormone levels were maintained, or increased somewhat, during the early stages of migration, but fell thereafter. In females the plasma T4 and T3 levels of salmon collected on the spawning grounds were at or below detectable levels of the assays. The changes in thyroid hormone levels are correlated with changes in plasma insulin, gonadotropin, gonadal steroid hormones, cortisol and vitellogenin levels measured in the same specimens.