There are many hormones working together, impacting each other to make dogs come in to and go out of the heat. While, no one is required to understand complicated interactions between them, there are three significant hormones influencing heat cycle in dogs.
If you want to understand estrous (heat) cycle in female dogs, basic understanding of how those hormones work is necessary.
Three important hormones for dogs in heat
During first days of the heat, there is a significant rise of a hormone, called estrogen. Humans also have similar (but not identical) hormone. It is responsible for swelling of the vulva and creates the bloody discharge from lining of the uterus to lubricate it. Blood drops coming from the vulva is one of the first signs of female dog in heat.
Estrogen keeps climbing steadily, usually reaches its maximum two days before beginning of the second phase of the heat and then decreases rapidly.
Other important hormone when dog is in heat is luteinizing hormone also known as LH. This hormone, similarly to estrogen, starts climbing at the beginning in the heat, but with much, much slower rate. Then it peaks suddenly, usually two days after the peak of estrogen, and then drops back down, and it causes the ovulation.
Because of that, measuring LH is one of the most precise ways to predict ovulation, which is important for breeders. Usually the whole process of LH peaking and getting back down takes about 24 hours, so when the first phase, proestrus, of the heat is about to end, LH must be measured every day. Unfortunately, unlike human “pee-on-a-stick” ovulation tests, dogs LH levels may be measured by blood tests only, and must be done at a clinic. When a LH peak occurs, it is considered ovulation will happen in next 24 hours. In some cases ovulation do not happen at all, even though female is in heat and LH did peak. This is considered as an abnormality in heat.
LH peak not only causes ovulation, but start climbing of the hormone called progesterone, as well. Progesterone makes the uterus receptive to implantation and supports the growth of the fetus. Measuring this hormone helps to tell the exact day of ovulation with greater precision, however, due to its price it is usually done if previous breeding had complications.
Progesterone usually starts to rise right after the peak of luteinizing hormone and reaches specific concentration when ovulating. In order to catch this moment, progesterone tests should also be done daily starting from when dog is about to be in standing heat. Progesterone is also used in medication to stop the heat cycle, as it, if we want to keep it short, stops production of estrogen.
Progesterone levels stay higher than normal for about two months, no matter if the bitch got pregnant or not. LH and estrogen hormones are backing down before the heat is over.
This post is a part of series about dogs in heat.