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

## TL;DR

This study explores how thyroid hormone levels in killer whales vary with age, season, and pregnancy, and how these variations may affect reproductive outcomes.

## Contribution

The study provides the first detailed analysis of thyroid hormone profiles in killer whales and links abnormal levels to pregnancy complications.

## Key findings

- Thyroid hormone levels in killer whales are influenced by age, with juveniles showing higher concentrations.
- Seasonal variations in thyroid hormones suggest thermoregulatory adaptations, with peaks in summer and declines in winter.
- Abnormal thyroid hormone profiles are associated with adverse pregnancy outcomes like stillbirths and abortions.

## Abstract

This study establishes the first information for serum thyroid hormones in killer whales, which are vital for regulating metabolism and growth. Unlike some cetacean species, killer whales showed no hormonal differences between males and females. Key factors affecting these hormones in killer whales include: (1) age where younger animals have higher levels to support rapid growth, (2) season with hormone patterns characterized by increased consumption possibly linked to generating body heat, and (3) pregnancy with hormone concentrations being relatively increased during early gestation and then decreasing towards late pregnancy. In addition, the study found abnormal levels are linked to complications including miscarriages and stillbirths. Overall, the results show that killer whale thyroid secretion is influenced by environment, age and reproductive status.

The study examined how season, age, sex, and pregnancy outcomes influenced serum total thyroxine (TT4) and triiodothyronine (TT3) levels in killer whales (Orcinus orca). 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.