Luteinizing hormone is a protein hormone that is released from a part of the brain, called the pituitary. Around puberty, or the beginning of a breeding season, LH is released in an episodic manner (in short pulses) after stimulation from another master hormone (gonadotropin releasing hormone or GnRH for short) in response to all sorts of cues such as- Environmental cues like: daylight length, rainfall and temperature. Social cues: like presence of a mate or other cycling females and/or physiologic cues: like attaining a certain body size supportive of reproduction. When LH is released into the bloodstream, it creates effects on distant tissues like the gonads (testes in males, and ovaries in females) to trigger the production and release of other important hormones. These hormones, in turn, by their level of production send signals back to the brain for either increased or decreased release of LH. Like a worker on the floor of the factory sending a signal to the foreman to speed up or slow down the delivery of raw materials to be manufactured into a product.
In addition, in the female cat (queen), LH is released in one large surge by mechanical stimulation of the vagina. This neuroendocrine reflex is what makes cat induced ovulators. That is, in the queen, ovulation usually only happens when she is mated by a tom cat. In about 30-35% of cases, spontaneous ovulation may occur.
What does LH do in the (queen) female cat?
LH is the initial signal sent from the pituitary to signal the release of hormones that will result in ovulation of multiple follicles (where the eggs are held in the ovary). Picture from Senger 2-11 and 8-2. The LH surge is directly proportional to the number of matings in the estrogen primed queen. Too few matings, especially early in the follicular phase, may fail to produce ovulation. The number of oocytes(eggs) ovulated is also related to the number of matings, with 4 matings/estrus being associated with a large number of oocytes released. LH surges occur within minutes of copulation. An ovulatory surge of LH is normally seen 24-27 hours after multiple matings, after the first 2-3 days of estrus. It is also responsible for the early development of the corpus luteum- the structure that develops in the ovary after ovulation of a follicle that will produce and secrete progesterone in quantities capable of maintaining a pregnancy. Senger 9-8 In the queen, not much is known about the factors related to lifespan of the corpora lutea. They seem to be relatively resistant to prostaglandins until after day 40, and anti-prolactin drugs seem to be capable of inducing their destruction as early as day 30. It is assumed, as in the bitch, that prolactin is a significant hormone assisting in luteal maintenance.
What does a high level of LH indicate?
There are 3 possibilities for this result:
1: This is an intact female experiencing a pre-ovulatory LH surge that will result in ovulation 24-27 hours later. Size of the LH surge is related to the number of copulations.
2: This is a spayed female which does not have enough estrogen or progesterone from her ovaries to suppress release of LH from the pituitary (foreman keeps sending out more raw materials because he can't hear the workers telling him to stop)
3: The ovary has a type of tumor that is interfering with LH hormone receptors causing sustained release of LH from the pituitary as the pituitary assumes the signal has not been received.
Other medical history, diagnostic tests and clinical findings can be used to sort these out.
What do you normally use LH for?
LH can be used to identify spayed/neutered animals or animals with gonadal dysfunction. As LH will be persistently elevated about 2 weeks after gonads are removed, this can be helpful in identifying a stray or found animal as spayed/neutered without the need for surgery, identification of retained gonadal tissue (cryptorchid males and ovarian remnant syndrome in females), potentially also for identification of a granulosa-thecal cell tumor of the ovaries in some bitches and bitches with testicular/ovarian failure such as intersex conditions or toxic injury to the gonads.
As the cat is an induced ovulator, serum LH is not used, as in the bitch, to predict time for breeding/insemination. Rather, it is assumed that sufficient number of breeding after the 2nd to 3rd day of estrus should result in ovulation. LH is also not used for prediction of date of queening (delivery) as factors involved in predicting this date can be complicated by the number of days over which the queen was bred and stressful conditions during delivery.
In case of identification of retained or functionally active gonadal tissue, one or more test may be needed depending on the animals' history and other clinical findings. Dr Sebzda can consult with you regarding your pet's case to determine what is expected in the individual situation.
How is LH tested?
LH assays come in semi-quantitative enzyme linked absorbent assays (ELISA) kits from Synbiotics. The LH test requires about 40 minutes for sample preparation and another 20 minutes for analysis. Not to worry though, you may always just have the pet come in for one of our experienced and caring technicians to draw the blood sample and have Dr Sebzda call you with the results at home (or any other contact info you provide), lessening the length of time you need to wait in the hospital.
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