The thyroid gland secretes hormones which are very important for development, growth, reproduction and metabolism. Sometimes the thyroid gland does not produce enough thyroid hormone and hypothyroidism occurs. Hypothyroidism is very common in dogs and many are diagnosed and treated with thyroid hormone supplementation. Hypothyroidism is diagnosed by collecting a good history, evaluating clinical signs, performing a complete physical exam, and measuring the concentration of thyroid hormones in serum. Generally this works very well, however measured concentration of thyroid hormones may be variable in female dogs, depending on the phase of the reproductive cycle that is operational when the sample is collected and analyzed. This variability may lead to inappropriate interpretation of thyroid concentration data and diagnostic errors.
Reproductive hormones vary consistently throughout the estrous cycle of dogs and may influence (increase or decrease) thyroid hormone secretion. If thyroid hormones are variable with estrous cycle phase, the interpretation of thyroid data needs to be adjusted to reflect these physiological interactions. The use of general, interpretive comments without attention to the reproductive status of intact bitches may lead to inappropriate classification of thyroid status. Animals may be incorrectly placed on thyroid supplementation, and unnecessarily removed from breeding programs. Alternatively, the diagnosis of hypothyroidism may be masked by thyroid hormones that are transiently elevated due to reproductive status. We wish to determine whether thyroid hormones and thyroglobulin status are variable in bitches at different phases of their estrous cycle to improve the accuracy of diagnostic testing for hypothyroidism in cycling bitches.