Hypothyroidism Spay/neuter in dogs was found to be correlated with a three fold increased risk of hypothyroidism compared to intact dogs. The researchers suggest a cause-and-effect relationship: They wrote: “More important [than the mild direct impact on thyroid function] in the association between [spaying and] neutering and hypothyroidism may be the effect of sex hormones on the immune system. Castration increases the severity of autoimmune thyroiditis in mice” which may explain the link between spay/neuter and hypothyroidism in dogs. Hypothyroidism in dogs causes obesity, lethargy, hair loss, and reproductive abnormalities.