How hormones regulate the heat cycle of female dogs

Changes in hormones, or the body’s signal molecules, are responsible for variations in your dog’s body and behavior during the heat cycle. Several female dog hormones work together, and their balance is what your dog’s body tell what to do.

German shepherd giving its paw to its human (in the golden rays of sunlight at the sunset)
Photo by Olena Kurashova.

If you want to understand the estrous (heat) cycle in female dogs, a basic understanding of how those female dog hormones work is necessary. In this article, we will look at the three most important hormones for the dog’s heat cycle. These are estrogen, progesterone, and luteinizing hormone.

Estrogen prepared body for implantation

During the first days of the heat, there is a significant rise of a hormone called estrogen. As the name implies, it has something to do with the estrous cycle, and one could say that it initiates it.

Humans also have estrogen, just like all vertebrates do. As a side note, an estrogen is actually a group of hormones of which the most known is estradiol. The list of estrogen’s functions is quite long; you can look it up at Wikipedia.

Estrogens are important in sexual development of females, but what regards to heat cycle, estrogen promotes swelling of the vulva and creates the bloody discharge from the lining of the uterus to lubricate it. This is when you notice the red discharge, which is the first sign of a female dog in heat. Don’t worry. Your dog is not bleeding. The lubrication of uterine is essential for later implantation of a fertilized egg.

Estrogen continues to climb steadily, usually reaches its maximum two days before the beginning of the second phase of the heat and then decreases rapidly. The peak of estrogen induces a sudden surge in another hormone.

Luteinizing hormone triggers the ovulation

This another hormone that regulates heat cycle in female dogs (and not only dogs) is luteinizing hormone (LH). During the first days of the heat, the hormone stays at the base level but suddenly surges dramatically and drops back to slightly elevated within a day. This peak is what causes ovulation, again, not only in dogs.

Besides ovulation, the surge of LH also induces production of corpus luteum, or, a yellow body. This yellow body develops in the ovaries after the egg leaves it. Its primary function is to promote and maintain pregnancy, which it does through a release of several hormones, including estrogen that you already know and progesterone which you will learn about next.

Progesterone assists implantation and maintains pregnancy

Progesterone is another of female dog hormones. It is mostly produced by the yellow body corpus luteum. It starts to climb right after the mentioned surge of luteinizing hormone. Progesterone makes the lining of uterus receptive for implantation and helps maintain the pregnancy.

If you are planning to breed your dog, measuring progesterone can become invaluable. As said, progesterone starts to climb right after the surge of luteinizing hormone. So, when the progesterone starts to rise, you can predict that ovulation will occur within two days. More importantly, three days after the ovulation is the most fertile time for dogs, which is critical information to time the breeding or artificial insemination.

However, progesterone levels are measured by blood tests. To catch the moment when it starts to climb, you should measure it daily (or according to your vet’s instructions) from the moment you first notice the signs of heat. Of course, the test has a price tag. Therefore, it is usually done when previous breedings have failed.

Progesterone level stays higher than usually about two months after the heat, regardless of whether the female dog got pregnant or not. Leutneizing hormone and estrogen, on the other hand, fall back to base levels well before the heat is over. If you are an owner of an intact female dog, you can find more information in our series about the heat cycle in female dogs.