🧠 Canine Sex Hormones and the Consequences of Spay/Neuter and Timing

Insights from Dr. Michelle Kutzler’s Presentation on Long-Term Health After Spay/Neuter (Gonadectomy)

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Many well-meaning dog owners opt to spay or neuter early, often encouraged by tradition or shelter policies. But emerging research—including work shared by Dr. Michelle Kutzler, a leading expert in theriogenology—invites us to revisit these decisions with long-term health in mind.

Here are a few takeaways:

Spaying and neutering may carry long-term risks when done without considering age, breed, and individual health. According to Dr. Kutzler, altered dogs show higher rates of issues like:

• Obesity
• Urinary tract problems
• Skin disorders
• Musculoskeletal challenges
• Behavioral changes
• Certain cancers

Why does this happen?

The removal of reproductive organs disrupts a key hormonal feedback system between the brain and gonads (called the hypothalamic-pituitary-gonadal axis). Without the ovaries or testicles providing feedback, levels of luteinizing hormone (LH) can become up to 30 times higher—a biological state we’re only beginning to understand fully.

What Is the Hypothalamic-Pituitary-Gonadal Axis?

And why it matters to your dog’s health and development

The HPG axis is a communication system between three major parts of the body:

1. Hypothalamus (in the brain)
2. Pituitary gland (also in the brain, just below the hypothalamus)
3. Gonads (testes in males, ovaries in females)

Together, this system controls sexual development, hormone production, and fertility—but it also plays a role in growth, behavior, immune function, bone density, and even cancer risk.

How It Works (Simplified)

Think of it like a hormone relay team:

1. The hypothalamus starts the process by releasing a hormone called GnRH (gonadotropin-releasing hormone).
2. GnRH tells the pituitary gland to release two more hormones:
   • LH (luteinizing hormone)
   • FSH (follicle-stimulating hormone)
3. LH and FSH travel through the bloodstream to the gonads, which then produce sex hormones:
   • Testosterone in males
   • Estrogen and progesterone in females

These hormones are responsible for:

• Sexual maturity and reproductive cycles
• Sperm and egg production
• Heat cycles and pregnancy
• Influencing behavior, metabolism, and growth

The HPG axis also works on a feedback loop. If there’s too much or too little hormone, the brain adjusts production to keep things in balance.

Why the HPG Axis Is So Important

• It regulates puberty – Controls when dogs reach sexual maturity.
• It influences bone growth and closure – Removing the gonads (spay/neuter) too early can delay growth plate closure, increasing injury risk.
• It affects brain development and behavior – Sex hormones interact with brain regions that manage fear, aggression, and learning.
• It supports immune health and cancer resistance – Hormones from the gonads help regulate immune system and cell repair.
• It maintains endocrine balance – Disruptions in the axis (like early gonadectomy) can lead to long-term hormonal imbalances.

Understanding LH Receptors and the Broader Impact of Spay/Neuter Decisions

Emerging research is shedding light on the presence of luteinizing hormone (LH) receptors beyond the reproductive system—including in the urinary tract, bones, adrenal glands, skin, and blood vessels. When LH binds to these receptors, it can trigger cell division and impact nitric oxide production, potentially influencing various body systems.

One of the key findings is that spaying and neutering may contribute significantly to obesity in dogs, primarily through two mechanisms: a decrease in metabolic rate due to reduced muscle mass, and a 20% increase in food intake driven by reduced satiety.

A notable concern involves the prevalence of urinary incontinence in spayed females. Spayed dogs experiencing incontinence tend to have higher LH receptor concentrations in their urinary tract, and some treatments aimed at lowering LH levels (like estrogen therapy or GnRH manipulation) have shown promise in restoring continence.

Additionally, large-scale veterinary data suggests urinary stones are three times more common in altered dogs, and roughly 20% of spayed bitches may develop coat changes, often resembling a soft, “puppy-like” texture due to hormonal shifts affecting the hair cycle.

The Hidden Orthopedic Costs of Spaying and Neutering

Recent research is shedding light on the long-term orthopedic consequences of early spay/neuter in dogs. Two key conditions—cruciate ligament rupture and hip dysplasia—are shown to be significantly more common in altered dogs, even when adjusting for factors like obesity or age.

🔹 Cruciate Ligament Rupture (CCL)
While many dogs are born with normal stifle joints, altered hormone levels post spay/neuter may increase laxity in the ligaments. In fact, one study of over 500 dogs found zero cases of CCL in intact dogs, suggesting a strong link. Just like in humans, reproductive hormones appear to influence ligament stability.

🔹 Hip Dysplasia

This condition involves joint laxity and misalignment of the hip socket and femur, eventually leading to arthritis. Spayed and neutered dogs are 1.5 to 2 times more likely to develop hip dysplasia compared to intact dogs. The lack of sex hormones—and the resulting rise in LH (luteinizing hormone) receptor activity—may disrupt normal joint development and maintenance.

Hormones, Behavior & Brain Health: The Role of LH After Spay/Neuter

When we think of hormones, we often focus on reproduction—but hormones like luteinizing hormone (LH) impact far more than fertility. LH receptors are found in the hippocampus and hypothalamus, two areas of the brain deeply involved in behavior, memory, and emotional regulation.

Behavioral Shifts After Spay/Neuter

After gonadectomy (removal of ovaries or testicles), LH levels remain persistently elevated. This can impact a dog’s behavior in multiple ways:

• Increased fear responses (storms, noises, separation)
• Heightened anxiety and timidity
• Owner- and stranger-directed aggression
• Submissive urination and stress illness
• Spayed females are up to twice as likely to show aggression compared to intact females—especially if already showing early behavioral concerns before surgery

While neutering may reduce inter-male aggression, the net effect on behavior is complex, and not always positive.

A Note on Treatment

Interestingly, Suprelorin (a GnRH agonist that suppresses LH production) has been shown to reduce anxiety and fear in neutered dogs—further confirming LH’s role in behavioral regulation.

LH and Cognitive Decline in Dogs (and Humans)

In aging dogs, elevated LH levels are linked to Canine Cognitive Dysfunction Syndrome (CDS)—a neurodegenerative disorder with symptoms similar to Alzheimer’s disease in people. These may include:

• Memory loss and confusion
• Night-waking or reversed sleep cycles
• Social disconnection
• Difficulty learning new things

Dogs who are spayed or neutered are at higher risk for both earlier onset and more rapid progression of CDS.

And this isn’t limited to canines. In humans, studies show that high circulating LH is associated with declines in cognitive performance and increased beta-amyloid plaque formation—a hallmark of Alzheimer’s disease.

LH and Cancer Risk in Dogs

Luteinizing hormone (LH) has been shown to influence the development of certain cancers in dogs, particularly through its interaction with hormone receptors. Mast cell tumors (mastocytomas)—the most common skin tumor in dogs—express high levels of LH receptors. Research indicates that spayed and neutered dogs are at higher risk for these tumors, especially in breeds such as Golden Retrievers, Vizslas, and large or giant breeds.

Hemangiosarcoma, an aggressive cancer arising from the lining of blood vessels (commonly affecting the skin, spleen, and heart), also exhibits functional LH receptors in vascular and smooth muscle tissues. This suggests a possible hormonal influence on its growth and spread.

Lymphoma, which accounts for up to 24% of all canine cancers, is significantly more prevalent in altered dogs—particularly neutered males, who are up to three times more likely to develop the disease. Studies show that 10–45% of cultured T-cell lymphomas express LH receptors, which are functional and may promote cancer cell survival and vascular invasion—increasing the cancer’s ability to spread throughout the body.

Beyond Reproduction: The Hormonal Role of Ovaries and Testicles

Ovaries and testicles are not just for reproduction—they are critical endocrine glands that support metabolism, musculoskeletal development, behavior regulation, and cancer resistance. Removing them eliminates important hormone-producing functions that impact overall health.

For some dogs, alternatives to traditional spay and neuter may be more appropriate. These options preserve hormonal balance while still preventing pregnancy:

• Vasectomy: The sperm-carrying tubes (vas deferens) are cut or sealed, leaving the testicles—and their hormone production—intact. This can be performed after the testicles descend or earlier using laparoscopy.
• Vasocclude® clips: A minimally invasive method involving a small scrotal puncture to block the vas deferens with a medical-grade clip.
• Hormonal downregulation: Medications like Deslorelin (Suprelorin®), a GnRH agonist, can temporarily suppress fertility in males without surgery.
• Hysterectomy (partial spay): In females, the uterus is removed while leaving the ovaries intact, preserving natural hormone production.

These options allow for better long-term hormonal health while still addressing reproductive control. Always discuss your dog’s lifestyle, health risks, and behavior with a knowledgeable veterinarian to determine the best approach.