What is progesterone?
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. Progesterone enhances the effects of estrogen on the female's brain to provide outward signs of estrus ("heat"). This is one of the reasons why a bitch's first heat can be silent or go unobserved when there was not sufficient progesterone to prime the brain prior to the secretion of estrogen. Progesterone, also like other steroids, can reduce the body's sensitivity to hormones like insulin that are helpful to glucose control- this is helpful to a pregnant mother with young, growing babies that require large amounts of energy as they develop into late pregnancy, but can lead to poor glucose control (gestational diabetes) in some bitches.
When is progesterone secreted?
Progesterone is not just elevated after ovulation and throughout pregnancy. In fact, in dogs, foxes and wolves, levels of progesterone start to climb around the time of the LH peak (the brain's signal to initiate ovulation). The sites where eggs will be released (antral follicles) from the ovaries actually start to change prior to ovulation and develop small amounts of luteal (Latin word for yellow) tissue. This luteinization of the follicles assist in follicular rupture and increases after ovulation as the cells in the ovary around the follicle, divide and continue to secrete large amounts of progesterone that will be needed to provide a uterine environment ready for embryos.
This means that progesterone will be produced in substantial amounts around the time of ovulation in the bitch and continue to rise into the first half of pregnancy.
How does this compare to other species?
Normally, other species, like farm animals, would release their eggs at ovulation and the tissues in the ovary left behind after ovulation, would start to form luteal tissue that would secrete substantial amounts of progesterone over the next 5-6 days. In the horse or cow, one would not expect large amounts of progesterone in the blood for at least 5 days after ovulation. Fig 7-2 from Senger.
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?
Well, levels of progesterone can be measured during the early estrous cycle of the bitch to assess the timing of the LH peak and the timing of ovulation. Usually the LH peak occurs at around the time progesterone concentration reaches 2.0 ng/ml and ovulation typically occurs at a time when progesterone concentrations are somewhere between 4.0 and 12.0 ng/ml. Progesterone levels can have a tendency to hover at lower levels for several days then jump up by 3.0 ng/ml or more in a single day which may be consistent with simultaneous ovulation of a number of follicles. Thus, it is actually much more valuable to follow the trend of progesterone levels than put stock into any one single value.
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.
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.
If estrogen is the hormone that induces signs of heat in the bitch, why can't you test for estrogen?
Well, you can in fact, check vaginal cytology to confirm that enough estrogen is being released to make the characteristic changes in the vagina of the bitch. The accuracy of serial vaginal cytology is however lacking, in that theses changes typically lag behind, at a variable rate, after the levels of hormone have been released. That means by the time you detect maximal vaginal cornification (flattening and loss of the cell's nucleus) on a smear, the peak of estrogen may have happened days ago, with no specific information on when exactly it happened. As estrogen is released in an episodic manner, it's awfully difficult to determine whether it is actually elevated or not, in a meaningful way. i.e. the levels of estrogen rise and fall so quickly that it would be random to catch an elevation in estrogen, and so we don't have a way to tell exactly what is a normal level of estrogen in an intact bitch going through heat. Progesterone however, is released in a continuous manner and its levels stay elevated for longer periods of time, making it much easier to interpret increases in this hormone and its timing relative to important events like ovulation and whelping.
So if my bitch starts to show signs of heat today, when do I start testing?
If we are starting from the initial signs of vulvar swelling, bloody vaginal discharge and /or attractiveness to male dogs: I would generally start with daily vaginal cytology to confirm presence of estrogen and follow the cytology until maximal cornification of the smears occurred. At that time (typically about 1/3 to 1/2 way into heat) I would start progesterone testing and check levels every day to every other day until the following events can be predicted: LH peak, ovulation, progesterone still continues to climb post-ovulation. Each breed and each individual can be different, so not one protocol fits every dog. However, bitches do have a tendency to have some consistency from cycle to cycle, so the protocol that fits may be used again in future ovulation timing procedures with good success.
Does progesterone testing have any other use other than timing of ovulation?
As mentioned earlier, it can be used for future prediction of a whelping date and it can certainly be used in cases of abnormal pregnancy to identify pregnancy loss or predict high risk for abortion. Consider also the following potential benefits: it can help increase litter size (if we can more accurately predict ovulation, we can identify the best days to breed and maximize the number of eggs fertilized before the eggs or sperm become to "old". It can permit us to use advanced technology like use of frozen semen shipped from other countries or dogs that were superior in performance but have died or been neutered. It can reduce the number of breedings used on a single cycle potentially lowering the risk for injury or uterine disease. It can be used in manipulation of future estrous cycles e.g. shortening a bitch's cycle to prepare for induction at a more desirable date, especially is she is currently performing in shows or sporting events. It can be used to detect silent heats in otherwise healthy bitches and be used to identify abnormalities of estrous cycle hormonal events in a work-up of infertility. This one little hormone has so many potential applications!!!!
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 chemiluminescence(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.
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 timing, whelping and early detection 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. If a progesterone level is needed more urgently e.g. prediction of impending pregnancy loss or planning a C-section, a semi-quantitative test from Synbiotics (OvuChek Premate 10) can be used. Dr Sebzda, DVM, DACT does all the interpretation of reproductive hormone assays.