Numerous reproductive abnormalities, including irregular interestrous period, anestrus, and infertility have been attributed to hypothyroidism. We previously documented normal fertility but higher periparturient pup mortality in bitches with hypothyroidism for a median duration of 19 weeks. The purpose of this study was to evaluate reproductive function in these same bitches after more prolonged hypothyroidism.
Fourteen multiparous bitches were studied. Hypothyroidism was induced in 8 dogs by administration of 1 mCi/kg 131I. Hypothyroidism was confirmed by finding serum T4 concentrations before and 4 hours after IV administration of human recombinant TSH that were < 5 nmol/L. Six bitches were euthyroid, untreated controls. Dogs were evaluated daily for signs of estrus and were bred by 1 of 2 males when serum progesterone >5 ng/ml. Ultrasonographic examinations were performed weekly beginning 21 days after ovulation to confirm pregnancy and monitor for fetal resorption. Interestrous interval, gestation length, strength and duration of contractions during whelping, time between pups, number of live pups and stillbirths, viability of pups at birth, weight of pups, and periparturient mortality were recorded. The Student's t-test was used to compare differences between control and hypothyroid bitches.
All hypothyroid dogs had signs of hypothyroidism present for at least 40 weeks. Breeding took place a mean of 56 weeks after 131I administration. No difference in interestrus interval or gestation length, or strength of contractions during whelping was noted between control and hypothyroid dogs. All 6 control and 4 of 8 hypothyroid bitches became pregnant. Fetal resorption was documented in 1 hypothyroid and 1 control bitch. The number of pups, puppy viability, and birth weight were significantly lower in hypothyroid dogs. Periparturient mortality, interval between delivery of pups, and duration of uterine contraction were greater in hypothyroid bitches. There were 8 stillborn pups from 4 litters, all from hypothyroid bitches.
Prolonged, severe hypothyroidism results in decreased fertility and increased periparturient mortality compared with euthyroid bitches. The results of this study show progression of reproductive abnormalities with more prolonged hypothyroidism when compared with a previous study of these bitches.