Thyroid hormones in canine pregnancy and lactation

Area of Science:

• Veterinary Endocrinology
• Reproductive Physiology
• Canine Health

Background:

• Thyroid function is crucial for mammalian fertility and fetal development.
• Limited research exists on how the reproductive cycle stage impacts thyroid hormone concentrations in dogs.

Purpose of the Study:

• To assess the influence of reproductive cycle stage and pregnancy on thyroid hormone concentrations in bitches.
• To evaluate established reference intervals for thyroid hormones in a female canine population.

Main Methods:

• Measured Thyroid Stimulating Hormone (TSH), total Thyroxine (tT4), and free Thyroxine (fT4) in 122 pregnant and non-pregnant canine reproductive cycles.
• Collected blood samples across six distinct stages: estrus, three points during pregnancy, lactation, and post-weaning (or equivalent non-pregnant stages).

Main Results:

• No significant differences in thyroid hormones were found between pregnant and non-pregnant dogs.
• Hormone concentrations varied significantly across the six sampling points (p < .01).
• TSH initially decreased then increased during pregnancy, peaking during lactation. tT4 and fT4 increased in early pregnancy and then declined.

Conclusions:

• Thyroid hormone concentrations in bitches change dynamically throughout the reproductive cycle and pregnancy.
• Established reference intervals for thyroid hormones may not be universally applicable and vary by sampling date.
• Cycle stage is a critical factor to consider when interpreting thyroid function tests in female dogs.