Variations in Circulating Thyroid Hormone Profiles Across Age, Sex, and Pregnancy Outcomes in Killer Whales (Orcinus orca) Under Human Care

The study examined how season, age, sex, and pregnancy outcomes influenced serum total thyroxine (TT4) and triiodothyronine (TT3) levels in killer whales (<i>Orcinus orca</i>). Total T4 and TT3 concentrations were quantified in 1513 serum samples collected voluntarily over ~40 years from 14 males and 24 females (ages 1-54) under managed care. Data were analyzed using LMM to determine the effects of age, sex, season, and pregnancy status (normal vs. abnormal outcomes). Age, season, and pregnancy significantly influenced thyroid hormone concentrations, while sex did not. Juveniles exhibited higher concentrations consistent with increased thermoregulatory needs and growth demands. Seasonal analysis showed TT4 peaked in summer and declined in winter suggesting thermoregulatory adaptation. Pregnancies with abnormal outcomes (abortion, dystocia, stillbirth) were associated with atypical thyroid hormone profiles; specifically, dystocia was linked to consistently low TT3/TT4, while stillbirths correlated with elevated late-term TT3. Females experiencing abortion showed decreased TT3 and TT4 during the late gestation. These findings suggest that in pregnancies with adverse outcomes, metabolic imbalances or transient hyperthyroid-like states may negatively impact fetal health. Consequently, in killer whales, variation in thyroid hormone levels may reflect a complex interplay between environmental adaptation, reproductive status, and underlying evolutionary physiology.