Reproductive effects of prolonged experimentally induced hypothyroidism in bitches.

Background: Hypothyroidism has detrimental effects on reproduction in females of many species. Studies of hypothyroidism in bitches are limited and results conflicting.

Hypothesis: Hypothyroidism interferes with reproductive function and health of offspring in bitches.

Animals: A total of 9 healthy mixed-breed bitches (control) and 9 mixed breed bitches with hypothyroidism induced by radioactive iodine administration.

Methods: Dogs in both groups were bred 20.9 ± 4.0 and 56 ± 7.6 weeks after radioiodine administration in the hypothyroid group and again after levothyroxine was administered for 37 ± 14 weeks to hypothyroid dogs. Measures of the estrus cycle, fertility, gestation, whelping, and pup health were evaluated at each breeding. Comparisons were made between hypothyroid and control dogs as well as within groups between times.

Results: Pregnancy was documented in all dogs in both groups at the 1st breeding, 4/8 and 6/6 untreated hypothyroid and control dogs, respectively, at the 2nd breeding, and 6/6 and 5/6 treated hypothyroid and control dogs, respectively, at the 3rd breeding. Periparturient mortality was higher and birth weight was lower in pups born to untreated hypothyroid dogs compared with control dogs or treated hypothyroid dogs. There was no difference in interestrus interval, gestation duration, breeding behavior, interval between birth of pups, or serum progesterone concentrations at any breeding between or within groups. Resolution of hypothyroidism reversed the detrimental effects of thyroid hormone deficiency on reproduction.

Conclusions And Clinical Importance: Hypothyroidism causes reversible periparturient mortality and low birth weight in offspring. Further investigation is necessary to determine if fertility is affected.