Is neutering dogs a zero-sum game?

Is neutering dogs doing more harm than good? This is not a question we ever thought we would be asking. As pet owners, we were told by our veterinarians, the animal shelters, and society at large that if we are responsible dog owners and truly love our dogs, we would neuter them at the earliest age possible. As veterinarians, we were taught in school that reproductive organs serve no purpose in dogs not used for breeding and should be removed as early as possible to prevent against reproductive diseases.

While there is still some truth in what we have been told in the past, there are now four decades’ worth of data and scientific papers to show that neutering male and female dogs does cause harm. While the majority of neutered dogs may not suffer from the adverse health effects of neutering, it is also true that the majority of intact male and female dogs do not suffer from the adverse health effects of keeping their reproductive organs.

Negative health effects of neutering

So, what are the adverse health effects of neutering? These can be categorized into cancerous conditions and non-cancerous conditions. The latter can be further categorized into metabolic, urinary, musculoskeletal, dermatologic, immunologic, and behavioural disorders. This list of all of the potential adverse health effects of neutering is too long to include in this blog, but the mechanism of action for all of these problems (cancerous and non-cancerous) comes back to an overproduction of a pituitary hormone following neutering.

Now, for a review of first-year veterinary reproductive physiology.The hypothalamus (in the brain) produces the main regulator of all reproductive activities—a hormone called gonadotropin releasing hormone (GnRH). This hormone (GnRH) travels through a special circulatory (portal) system within the brain to the anterior pituitary gland, where it induces the production and secretion of a gonadotropic hormone—luteinizing hormone (LH). In the intact male and female dog, LH travels through the systemic circulation to bind to its receptors, where it elicits a variety of responses. In the gonads, LH binds to its receptors to induce the production and secretion of testosterone (males) and estradiol (females). These steroid hormones then travel back through the systemic circulation to negatively feedback on the production of GnRH and LH. In the intact dog, circulating LH concentrations are below 5 ng/mL (and often below 1 ng/mL, which is the level of detection for most assays).

In neutered dogs, there is no negative feedback since the gonads have been removed. As a result, LH concentrations soar to supraphysiologic concentrations of up to 90 ng/mL—and these high concentrations persist for the remainder of the dog’s life. Since there are no gonads for the LH to bind to, LH will bind to other cells/tissues that have LH receptors. These tissues include (but are not limited to) the urinary tract, bone and ligaments, thyroid, adrenal and pancreatic cells, vascular endothelial and smooth muscle cells, and lymphocytes (both B- and T-lymphocytes).

What is also interesting is that the cells in neutered dogs have more receptors for LH, making them even more responsive to the high LH in circulation. Binding to receptors in these cells, LH can block the normal function of the cell (e.g. to reduce the amount of thyroid hormone produced by thyrocytes); increase the function of the cell (e.g. to overproduce cortisol from the adrenal gland); stimulate nitric oxide release to induce smooth muscle and ligament relaxation (e.g., resulting in urinary incontinence or joint instability); stimulate cell proliferation (e.g. in the case of cancer), etc.

What does this mean for vets and dog owners?

So what are we supposed to do now? Should we neuter dogs and if so, when and for what specific reason?

There are more humane methods of preventing unwanted birth litters than amputating the dog’s reproductive tract. Responsible dog owners keep their intact male and female dogs confined to prevent pregnancies from occurring. Or, if an unwanted pregnancy does occur, there are safe, medical ways to terminate the pregnancy. In addition, there are safe, long-term hormonal implants that prevent fertility for several months to years, depending upon the age when they are administered and the size of the dog.

I think the bigger question on all dog owners’ and veterinarians’ minds now is - what do we do with all of the neutered dogs. Should we be prophylactically lowering LH concentrations to prevent them from developing these LH-related diseases? When we are treating these LH-related disease using other medication, should we also be lowering LH concentrations? More research and clinical trials are needed to answer these questions.

But one question has been answered- neutering dogs is not a zero-sum game. It will cause harm. Whether it is more harm than good is still up for debate.