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This study evaluated circulating concentrations of thyroid hormones in relation to age, sex, pregnancy status, and perinatal loss in bottlenose dolphins (Tursiops truncatus) under human care. A total of 373 blood samples were collected from 60 individual dolphins housed at nine aquariums/oceanariums. Serum concentrations of total and free thyroxine (T-4) and triiodothyronine (T-3) were analyzed with commercial RIA kits validated for use with dolphins. While the effect of age was indicated by higher (P < 0.0001) concentrations of total and free T-4 and T-3 in juveniles than adults, the effect of sex on thyroid hormones was inconclusive. The effect of pregnancy was indicated by higher (P < 0.035) total and free T-4 and T-3 during early pregnancy compared to non-pregnancy. For both successful and unsuccessful pregnancy outcomes, maternal concentrations of thyroid hormones were highest during early, intermediate during mid, and lowest during late pregnancy (P < 0.07 to P < 0.0001). Compared to live and thriving births, concentrations of total and free T-4 and total T-3 were lower (P < 0.08 to P < 0.001) in dolphins with perinatal loss. Lower concentrations ranged from 10% to 14% during early, 11% to 18% during mid, and 23% to 37% during late pregnancy. In conclusion, the effects of age, reproductive status and stage of pregnancy on thyroid hormone concentrations are necessary factors to take into account when assessing thyroid gland function. Since perinatal loss may be associated with hypothyroidism in dolphins, analysis of serum T-4 and T-3 should be considered for those dolphins that have a history of pregnancy loss. (C) 2013 Elsevier Inc. All rights reserved.