Canine reproduction: about hypothyroidism and infertility

 

I recently got the following question during one of my lectures on canine infertility. “You touched on uterine diseases, ovarian diseases, infectious diseases that can impact the bitch’s fertility… But what about hypothyroidism? Why didn’t you mention it?” My answer: “Good question! Well, because I truly think that if the animal really suffers from hypothyroidism, infertility will usually not be the first reason with we’ll see it in consultation.” Surprised? Let me elaborate here.

 

  • Thyroid hormones: what are they for?

In our dogs – and in fact in all mammals-, the thyroid gland is located in the cervical area. And the hormones this little gland produces – aka thyroid hormones, but you might also hear about T3 or T4- play an important role in the good functioning of our body. We often consider them as “the hormones of the basal metabolism”. Which just mean they are active somehow in nearly all the main organic functions.

When a dog suffers from hypothyroidism, it means that there is a decrease in its thyroid hormones blood concentration. Because of their wide variety of actions, you now understand why there are several different clinical expressions. The following clinical symptoms are the most commonly observed and will raise the veterinarian’s suspicion:  

-          Skin issues – certainly the most frequent-: hair loss (often described as “hairs only on the head and the legs”), hyperpigmentation, seborrhea, cutaneous edema

-          Weakness, lethargy, difficulty to move

-          Bradycardia

-          Weight gain

  • Thyroid hormones and reproduction?

There is a dialogue between the thyroid hormones secreted by the thyroid glands and the different organs of the reproductive tract as well. The same way hypothyroidism can influence the other organic functions, it can also impact reproduction. And in all textbooks of canine repro that I read over the years, there is always a paragraph somewhere mentioning how hypothyroidism could cause of infertility. I have no doubt this is true. I however think the real question we should ask ourselves is the following: “ is it something common?”. And there are interesting data we can find on that last point in the scientific literature.

  • Hypothyroidism, an uncommon cause of infertility

It indeed seems that hypothyroidism is NOT a common cause of fertility. I picked two studies published in peer-reviewed journals to illustrate this:

-          First one was conducted by my former team at the Alfort Veterinary College (Paris, France) – click here to see the abstract : thyroid function was evaluated in 204 dogs from 5 different breeds, some of which considered as “predisposed to hypothyroidism” (we could have a big debate here too I guess, but we won’t follow this path today!). Conclusion: no difference in thyroid hormone concentration between fertile and infertile dogs.

-          In the second study, they experimentally induce hypothyroidism in 9 bitches and study how it could influence their reproductive function – click here to read the abstract. Conclusion: I quote “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. ”

To sum up: hypothyroidism CAN cause infertility, but breeding dogs which suffer from this disease will usually exhibit other clinical signs as well. If you’ve read the second abstract I shared with you, you might have seen that they also discussed whether or not hypothyroidism might have an effect during pregnancy and on the neonates. Data are still scarce but this could be something to focus on in cases of neonatal mortality. But that’s another story…

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