Lack of effect of trimethoprim and sulfadiazine in combination in mid- to late gestation on thyroid function in neonatal dogs.

Four pregnant bitches were randomly selected and were given 25 mg sulfadiazine kg-1 and 5 mg trimethoprim kg-1 orally once daily, starting on day 29 of dioestrus until parturition. Five untreated pregnant bitches were used as controls. At the end of the treatment period (parturition = week 0), one to three pups of each litter, treatment and control group, were randomly selected and blood was obtained by cardiac puncture for serum thyroxine (T4) and triiodothyronine (T3) concentrations. These pups were then killed and the thyroids were harvested and examined histologically. This protocol was repeated three times at weekly intervals. There were no significant differences in mean T4 and T3 concentrations between treated and control groups at any time during the study. Mean concentration of serum T4 and T3 did increase significantly over the 3 weeks of study. Histologically, no significant differences in thyroid structure were observed between treated and control groups. The numbers of stillborn or weak pups did not increase in the treatment group. No differences were observed in the length of gestation between the treated and control groups. Administration of sulfadiazine and trimethoprim does not affect the thyroid gland in the neonate.