Q & A: Testosterone Stimulation Test for Diagnosis of Cryptorchidism

I just read your last blog post on the "Best Test to Determine if Cat is Cryptorchid or Castrated," which I found interesting. My patient is a 1.5 year old male DSH presented for evaluation of inappropriate urination, and strong smelling urine. The cat was adopted into a multi-pet house from a rescue organization. His previous history is unknown, but the cat is supposedly neutered.

On my exam, the cat was quite fractious. He has no palpable testicles, but I was not able to examine his penis due to his nature. However, I was able to get a blood sample, and a basal testosterone concentration was 87 pg/mL. According to lab's reference ranges, this value is between cryptorchid and male-castrated levels.

My questions:

1. How do I interpret this testosterone level?
2. Is there another test I can do to determine if this cat has been castrated or not?

My Response:

Yes, in cats like this one there are provocative tests we can do. Like I said in my last post, however, I rarely do these  tests—I check the penis for the presence of barbs or backwards-pointing spines (1). These penile barbs indicate that testosterone is present in a male cat's body. These penile barbs will regress within a month after successful castration. This "bioassay" for testosterone is the cheapest and best kind of assay.

However, if you or the cat owner want a testosterone value to support this "bioassay" evidence, or — as in this cat— you can not examine the cat's penis without sedation, determination of testosterone concentrations can be helpful. Because circulating testosterone is released in a pulsatile manner, a single, basal sample is generally not very helpful —if you get a high value, that's diagnostic for testicular tissue, but if the testosterone value low to low-normal (as in this cat), it tells us nothing.

In male cats, testosterone stimulation testing can be done with administration of either:

1. Human chorionic gonadotropin (hCG), which closely mimics the effects of luteinizing hormone (LH), or; 
2. The hypothalamic hormone, gonadotrophin-releasing hormone (GnRH), which stimulates the pituitary secretion of LH and FSH.

Test Protocols and Interpretation of Results:

• To perform an hCG challenge test in a male cat, collect blood samples for serum testosterone measurement before and 4 hours after administration HCG at the dosage of 250 μg, IM (2,3).
• To do a GnRH stimulation test,  collect blood samples for serum testosterone before and 1 hour after administration GnRH at the dosage of 25 μg, IM (3). Use of GnRH (Cystorelin, Merial) for stimulation testing has advantages over hCG, as it will not induce any antibody formation, the sampling time is shorter, and it is very inexpensive.
• After injection of either of these provocative agents, a 2-fold increase in a basal testosterone level is diagnostic for the presence of retained testicular tissue.

References:

1. Aronson LR, Cooper ML. Penile spines of the domestic cat: their endocrine-behavior relations. Anat Rec 1967;157:71–78.
2. Schille VM, Olson PM. Dynamic testing in reproductive endocrinology. In: Kirk RW (ed): Current Veterinary Therapy X.  Philadelphia, WB Saunders, 1989, p 1282.
3. Johnston SD, Root Kustritz MV, Olson PNS. Disorders of the feline testes and epididymes. Canine and Feline Theriogenology, Elsevier, Philadelphia, 2001, p 521.