What is progesterone? (Feline)

Progesterone is a hormone produced by the ovaries and placenta that helps to maintain pregnancy. It's main effects on tissues inside the body include: induction of an elaborate network of glands (endometrial glands) in the uterus to help provide nutrition to the early conceptus(baby) and become the maternal side of the placenta (connection between baby and mom). During pregnancy, it helps keep the uterine muscle layers relatively quiet so as not to disrupt a pregnancy. It also helps suppress the mother's immune response to its "foreign" baby as the baby grows and develop it's own immune system. Progesterone also provides the stimulus to development of the glandular portions of the mammary glands (breast tissue)- it along with estrogen and other hormones, produce the changes to breast tissue after puberty, throughout pregnancy and during nursing to allow these tissues to produce milk for babies.

When is progesterone secreted?

Progesterone starts to rise after ovulation and throughout pregnancy. The sites where eggs will be released (antral follicles) from the ovaries start to change after ovulation and develop small amounts of luteal (Latin word for yellow) tissue. This luteinization results in secretion of large amounts of progesterone that will be needed to provide a uterine environment ready for embryos.

Unlike the dog, progesterone may not drop off sharply prior to ovulation. Some queens have even delivered kittens when progesterone was still elevated around 5 ng/ml. Other queens delivered over several days when progesterone reached baseline values. Earlier research showed that queen's did not need progesterone to maintain pregnancy after 40 days, as progesterone levels could drop to baseline. More current studies dispute this and would seem to suggest that progesterone is required for the duration of pregnancy.

How does this compare to humans?

Humans produce progesterone from sites of ovulation that form luteal bodies after ovulation as well. This happens about halfway into a women's menstrual cycle and at the end of the lifespan of the luteal body, as progesterone drops off, the menstrual cycle (period) begins. Fig 7-10 from Senger

How does this information benefit the breeder and the veterinarian?

Progesterone can also be used to confirm that ovulation did take place- levels over 15.0 ng/ml can be used retrospectively to confirm ovulation, but does not help in future planning of a breeding of that cycle. It can however, determine ovulatory from non-ovulatory cycles. On a cautionary note, as mentioned previously, some queens may have low levels of progesterone after day 40 of pregnancy and so a low level at day 40 or more should not be assumed to be associated with lack of ovulation. Progesterone ideally should be tested between 5 and 40 days post mating to detect ovulation.

Progesterone can also be used to estimate the date of whelping. Fortunately for veterinarians, dogs seem to be very consistent in the timing of whelping. On average, whelping occurs about 63 days from the point of ovulation and 65 days from the point of the LH peak (both of which are estimated by progesterone levels as mentioned above). The luteal bodies seem to have a finite lifespan and although it is not completely understood what triggers their demise, as the progesterone levels fall below 2.0 ng/ml, whelping occurs within 48 hours. A very useful piece of information in predicting a bitch's whelping, whether an abortion is about to occur, and in confirming termination of a pregnancy.

How should you manage the breeding of a queen in estrus?

If the queen is planned to be bred naturally by the tom, the queen should be brought to the tom's territory when she has been showing signs of estrus for 2-3 days. A certain amount of estrogen is needed to prime the queen's brain to obtain a good surge of LH on mating. Vaginal cytology can help to determine if the queen is in heat, but cannot distinguish the beginning vs. the end of heat. Observation of mating should include witnessing an "after-reaction" in the queen, which normally includes a cry from the female, aggression shown toward the male (hissing, attempt to strike), rolling and genital licking. This after reaction is suggestive of the necessary coital stimulus to induce an LH surge. Lack of these signs should prompt the breeder to observe the pair again over the remainder of estrus as actual copulation (and vaginal stimulation) may not be taking place.

If the pair are not willing to breed or fail to show signs of successful mating (incl. after-reaction), then if the queen is in estrus, an injection of a hormone to induce ovulation may be given, and semen (either collected fresh or stored frozen) can be used to inseminate the queen 27-36 hours later. Artificial insemination requires training a tom to be collected from a teaser queen in heat (may take up to 2 weeks) or electroejaculation while under general anesthesia. Insemination of the queen can be done with a vaginally inserted probe in the estrual queen or placed directly into her uterus while the queen is under anesthesia after ovulation has been induced. Estrus can also be artificially induced in the seasonal polyestrous queen, using a variety of different drug protocols to co-ordinate the timing of events to facilitate assisted breeding.

How can I tell if the queen is pregnant from her mating?

A progesterone test after 5-40 days from the last mating can only suggest ovulation, not confirm pregnancy. Ultrasound of the queen 20 days later can permit identification of a pregnancy. Use the hormonal assay relaxin can also be used to confirm pregnancy after 25 days gestation. Relaxin comes from the placentae of the developing kittens and is thus a pretty specific test for pregnancy, but it does not show up in significant amounts until after 25 days gestation and can remain elevated several days after abortion. So it is a little less sensitive- meaning, a negative test run too early in gestation may not be negative and should be redone at a later date.A negative test run later into gestation is definitely negative. A positive test may not be positive if the litter is dead or lost recently.

Can you use any form of progesterone testing?

There are many forms of progesterone tests commercially available nowadays. Some tests are quantitative, meaning they give you a specific numerical value to the level of progesterone detected, and some and semi-quantitative, in that they don't give you a specific value but give you a range of values in which the sample fits best.

Of the quantitative tests, there are different methodologies used: radioimmune assay (RIA)- considered the "gold standard" of reliable testing of progesterone levels, enzyme linked absorbent assay (ELISA) and chemiluminescent(Immulite). These tests use varying technology to measure levels of progesterone in blood samples. There are a few limitations to their use. The blood samples should be assayed at the same time each day, as steroid hormone levels can fluctuate throughout the day. The blood samples must be allowed to clot and the serum separated from the clot within 2 hours, without the use of a serum separator gel. The gel in serum separator tubes can contain an ingredient that artificially lowers the level of progesterone detected. Plain red top tubes can be used for this purpose. The tests are normally run by a laboratory so, in most cases they yield information within 24 hours, but they are not immediate. They can be more expensive in some cases. There is also some variability in levels of progesterone detected between different assays- meaning one should never try to compare progesterone levels assayed by RIA with another method like Immulite, expecting them to be comparable. It is always best to stick to one method and follow the trend.

Of the semi-quantitative tests: most employ a color change to indicate whether the progesterone level falls into a low, intermediate or high test range. They can be convenient for immediate assay, although they are time-consuming in clinic, they do not provide a single numerical result, they can be somewhat difficult to interpret when the value of progesterone is intermediate to 2 of the ranges listed, and the lower and upper limits may be too high or too low to be of value in all cases. We normally do not use semi-quantitative tests in cats, as there is no practical reason for their use and ours are not validated for use in the cat.

Any progesterone test used should be validated for the species used and one should always check with manufacturer of the assay for this information prior to its use. Sometimes the cheaper test isn't so cost effective- especially if it's inaccurate!

Here at Newport Harbor Animal Hospital, we use Antech laboratory's RIA assay for progesterone levels and have great success in interpretation of progesterone for ovulation detection and diagnosis of ovarian and uterine pathology. Our technicians are trained in correct blood sampling technique and strive to get accurate samples for your pet in a safe and gentle manner. They are available 7 days a week during office hours. All of our results are available the next morning as Antech runs the assays overnight. Please note however, that while samples can be drawn for interpretation any day of the week, our laboratory does not run samples on Sunday evenings, so samples drawn on Sunday, will be frozen and then assayed Monday night to deliver results by Tuesday a.m.

Dr Sebzda, DVM, DACT does all the interpretation of reproductive hormone assays.

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