Immediate Effects of Postpartum Hypothyroidism and Long‐Term Effects of Neonatal Hypothyroidism on Maternal Behavior in Rats

Thyroid hormones are critical for brain development, and numerous studies have reported that hypothyroidism in neonatal life results in long-lasting changes in activity, cognition, and social behavior. However, most rodent developmental hypothyroidism paradigms induce hypothyroidism in the dam as well, and these enduring behavioral impacts may be partly mediated by changes in the maternal care pups receive. The goals of this study were to determine whether postpartum hypothyroidism alters maternal care and whether the maternal behavior of the pups experiencing postnatal hypothyroidism is altered when they have their own pups. We induced hypothyroidism using methimazole (MMI; 0.025%) in the drinking water from P0 to P21, and we measured maternal behavior using 30-min observations every 2 days for 2 weeks; these were followed by a pup retrieval test. When female offspring from these litters reached adulthood, we mated them with control males and repeated the maternal behavior observations and retrieval tests once pups were born. There were no largescale deficits in maternal care in either generation; instead, both groups of MMI-treated females retrieved faster and engaged in more nesting behavior than controls. These results suggest that the effects of postnatal hypothyroidism on early brain development are not likely caused by maternal neglect.