CHAPTER 9Endocrine Tests
To provide some insight into the growth rate of routine endocrine diagnostics in equine practice the following time line may be of interest. In 1984 BET Labs of Lexington, Kentucky, was launched to provide a commercial endocrine diagnostic service for veterinarians. In that first year approximately 6,000 hormone assays were performed, mostly for progesterone and predominantly for veterinarians in the Lexington area. Twenty years later this laboratory performs approximately 60,000 assays annually, with progestins still most commonly assayed, for veterinarians throughout North America. When accompanied by a United States Department of Agriculture (USDA) import permit, it is possible for our laboratory to receive serum for assay from almost any part of the world. Application of endocrine diagnostic testing in other countries has been greatly limited due to lack of knowledge and economics. For example, BET Labs do Brasil has been in existence for over 5 years in Rio de Janeiro, Brazil, and performs less than 1,000 assays per year in equine patients, even though the number of horses in Brazil is about equal to the number in the United States. The Brazilian dog owner is much more willing to pay for endocrine diagnostics than the horse owner.
PROGESTINS
In the pregnant broodmare progesterone is present only through 150 days of pregnancy, and thereafter the predominant progestins are 5α-pregnanes and 17α-hydroxy progesterone. The ovaries are no longer needed after 100 days of pregnancy to support pregnancy; thus only progestin concentrations during the first 100 days of pregnancy will be addressed in this chapter. In 1975 Holtan et al1 reported that progesterone became very low, dropping to levels below 2 ng/ml by 180 days of pregnancy, and remained low until 30 days before parturition, at which time levels rose dramatically until the mare foaled. By 1990 Holtan et al2 found that by using more specific assay techniques, including gas chromatography and mass spectrometry, progesterone was not detectable during the last half of pregnancy; however, progestins were detectable during this time and consisted predominantly of 5α-pregnanes. These progestins were actually what was rising before parturition. Recently it was suggested that 5α-pregnane-3, 20-dione (5αDHP) may be more biologically important than progesterone in late gestation in the mare.3 This importance is related to its greater binding by uterine P4 receptors than P4 or other progestagens and that its concentration rises during the last 20 to 30 days of gestation, falling rapidly during the last few days before birth.4
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