What is estrogen? (Canine)

What is estrogen? (Canine)

Estrogen is a steroid hormone produced mainly from the ovaries, but can also be produced from the adrenal glands and placenta (tissue that communicates between baby and mom during pregnancy).

When is estrogen produced?

Estrogen is produced in significant amounts during the first part of a bitch's estrous cycle (heat). It can also be produced in larger amounts towards the end of pregnancy (from the placenta) and from estrogen secreting tumors (e.g. granulosa cell tumor of the ovary and Sertoli cell tumor of the testis).

Estrogen can also sometime be given to dogs-e.g. estrogen given as a mismate shot (no longer recommended form of treatment) or inadvertently by owners that may be on hormone replacement therapy (transdermal creams/gels) or accidental ingestion of certain forms of birth control.

What does estrogen do?

Estrogen has many roles:

1: It is involved in the growth, maturation and release of oocytes (eggs) from the bitch's ovaries.

2: It prepares the lower reproductive tract for breeding, by thickening and lubricating the lining of the vagina. It promotes growth, increased blood supply, fluid accumulation in the endometrium of the uterus and supports migration of white blood cells into the uterine lumen to help defend it from invading pathogens.

3: It assists with motility and transport of male and female gametes (eggs and sperm) towards each other and transport of the embryo toward the uterus at the correct time.

4: It assists with preparation of the reproductive tract for delivery of the fully developed fetus (lubricates the vagina and softens the cervix) and aids in other hormones' effects to cause muscular contractions of the uterus.

5: It is involved in growth and development of the mammary tissue through puberty.

6: It affects the dog's behavior, to make her both more attractive and receptive to male dogs.

What are the clinical signs of elevated estrogen?

Normally one sees swelling of the bitch's vulva, a bloody vaginal discharge, and increased attractiveness towards male dogs (Note: not necessarily increased receptivity). If you touch the skin on either side of the bitch's tail (perineum), she may hold her tail off to one side -known as flagging


And sometimes anemia (red blood cell loss) will be seen in bloodwork.

In a pronounced case of estrogen sensitivity (during a shorter period of time), there may be significant swelling of the vaginal mucosa, such that it protrudes from the vulva appearing as a pink "doughnut- shaped" or "teardrop shaped" mass. It is referred to as vaginal prolapse or vaginal hypertrophy. This is especially common in breeds like the Boxer and English bulldog and usually accompanies estrus or precedes whelping. In severe cases, it may interfere with natural breeding and whelping and necessitate surgical intervention.

How do you test for estrogen levels?

Normally, outward clinical signs (as mentioned above) are already supportive of increased estrogen. Vaginal cytology is another useful test to detect the effects of estrogen on the bitch's vaginal tract. This can be helpful for cases where the outward symptoms are subtle. A swab of the anterior portion of the bitch's vagina (past the common area of urinary/reproductive tracts) yields more flattened cells with no nuclei, at the height of estrus. These are referred to as cornified epithelial cells and resemble "corn flakes" when viewed under the microscope. Here are some examples of cells seen on vaginal cytology (from LSU therio website)

No information about the exact quantity of estrogen in the bloodstream is derived from this testing. It is rather, a retrospective look at the cumulative effects of estrogen over the past several days.

Why can't you do a blood test for estrogen levels?

Well, you can sample serum for hormone levels, but as estrogen is released in a pulsatile fashion, at any given time, it may be high, intermediate or low and not necessarily representative of what's been happening for the last few days. The levels of estrogen are also very low overall compared to progesterone, making it more expensive and difficult to test. Finally, the peak of estrogen is usually reached late in proestrus, when the bitch is still non-receptive to the male. During the bitch's fertile period in estrus, estrogen levels are typically basal. Progesterone see what is progesterone is a much more useful test for timing of ovulation.

My dog has been spayed, but is showing clinical signs consistent with elevated estrogen. How is this possible?

Most likely, the dog is exposed to another form of estrogen from external sources. Hormone replacement therapy in menopausal women is often prescribed as transdermal creams or gels. It is very easy to a pet to become exposed to these hormones if they are applied to an area of the body that the dog may have contact with afterward e.g. inside of a leg or arm. Ingestion of certain forms of birth control or other estrogenic substances can be a cause as well. In rare cases, estrogen may come from the pet's food or water supply (usually more than one animal in household involved or new batch of food).

In some cases, a little piece of ovary may be left behind post-spay surgery. This is referred to as ovarian remnant syndrome. The bitch still demonstrates estrus although cannot become pregnant if the uterus was removed. In some countries, outside of North America, it was popular at one time, to harvest a piece of ovary and suture it into the outside surface of the stomach. It was believed that since the blood supply would drain mostly to the liver and not directly into systemic circulation, most of the nuisance symptoms of heat would be avoided and it would leave the bitch with some hormones to prevent problems like estrogen-dependent incontinence later in life. It has since been demonstrated that the omentum can re-vascularize this left over tissue and cause the estrogen to be released systemically both resulting in symptoms of heat at varying intervals and still can result in increased rates of mammary cancer and may even result in increased risk of ovarian cancer in the ovarian remnant. Obviously, we do not recommend this surgery.

There are tests that can be performed to diagnose ovarian remnant syndrome prior to surgical exploration (LH assay, progesterone tests and abdominal ultrasound). Surgical exploration of confirmed cases and removal of the retained ovarian tissue is advised in all cases as the remnant is at increased risk for neoplasia.

Our Pet Health Mission

Our mission at Newport Harbor Animal Hospital is: "To provide the highest quality veterinary care for our patients and the best service for our clients. Our goal in every case is a healthy pet and a happy client."

Blog Category: