Progesterone in cats and dogs

What it shows

Before ovulation progesterone is low. It starts to rise at about the time of the LH surge — the signal from the brain that makes the ovaries release eggs — and keeps climbing after ovulation. Then it stays high through the phase that follows, called diestrus, in a pregnant and a non-pregnant female alike.

So one number says little; a series says much more. In a dog in heat, vets take samples a few days apart to find the first clear rise above baseline and later to confirm ovulation. No single value defines a stage of the cycle, and bitches differ a lot in timing. How the result is used for planning is a conversation with the vet.

The test also helps to time a planned caesarean and, together with AMH, to look for ovarian tissue left after spaying.

Higher than reference — what it may be a sign of

• Ovulation has happened. After ovulation and through diestrus a high value is expected in every intact female; it does not mean pregnancy, because the levels are similar whether the female is pregnant or not.
• Ovarian tissue in a spayed female. A value above 2 ng/mL about two to three weeks after ovulation supports an ovarian remnant. It is read together with AMH: high progesterone can point to a remnant even when AMH is negative.
• Medicines that contain progestogens. They have to be ruled out before a high result is put down to the ovaries.

Lower than reference

A low value is normal outside the luteal phase — in anoestrus and early in heat — and in a female without ovaries. In a dog in heat, a level that never rises suggests that ovulation has not happened. In late pregnancy the level falls below about 2 ng/mL roughly a day before whelping, which is why it is watched near term. Low progesterone during pregnancy (luteal insufficiency) has been described in dogs but is poorly documented; a low result in a pregnant female is a reason to talk to the vet, not a diagnosis.

Look at together with

• LH (see AMH and other sex hormones) — measuring both LH and progesterone gives the most accurate picture of ovulation.
• Vaginal cytology — progesterone bands are meant to be read with the smear.
• Relaxin and ultrasound — they, not progesterone, answer “is she pregnant?”.

Units and conversion

Progesterone is reported in nmol/L (on Russian forms нмоль/л) or in ng/mL. One ng/mL equals 3.18 nmol/L, so 5 ng/mL is about 16 nmol/L. The method matters too: laboratory chemiluminescent assays are considered the reference, while in-clinic semi-quantitative kits are least accurate at about 1.5–10 ng/mL — exactly where the first rise happens. Results of different methods are not interchangeable, so a series is best done by one method. In the app each row keeps the unit printed on the form; the chart converts to one unit.

Reference range

The interpretation is on your lab’s form, and for progesterone it is often not a single reference range: some laboratories print none, because the expected value depends on reproductive status and the stage of the cycle, and give interpretation bands instead. As a rough guide, values below about 1 ng/mL (about 3 nmol/L) are typical before the LH surge, and the peak in diestrus ranges widely, from about 15 to 80 ng/mL.

Cats and dogs

Sources

1. Merck Veterinary Manual — Breeding Management of Bitches (professional version)
2. Whitbread T. J. Serologic Test Kits (canine ovulation timing — progesterone; LH). Merck Veterinary Manual, professional version
3. Michigan State University Veterinary Diagnostic Laboratory — Endocrinology: Assessment of Reproduction
4. Cornell University Animal Health Diagnostic Center — Canine Reproductive Function Tests
5. Prenatal Care of the Bitch and Queen (review, PMC)
6. Merck Veterinary Manual — Ovarian Remnant Syndrome in Small Animals (professional version)