Side Effects of Spaying a Dog: The Hormonal Changes Vets Don't Explain

At a Glance: What Pet Parents Need to Know

Spaying is one of the most common surgeries performed on dogs in the United States, and for good reason. It prevents unwanted litters, eliminates the risk of pyometra (a life-threatening uterine infection), and can reduce the likelihood of certain reproductive cancers. But what most veterinarians do not fully explain during that pre-surgery appointment is what happens to your dog's hormones after the procedure, and how those changes can quietly reshape her health for the rest of her life.

When the ovaries are removed during a spay, your dog loses her primary source of estrogen and progesterone. These are not just "reproductive hormones." They regulate metabolism, protect joints and bones, influence mood and anxiety levels, support coat quality, and play roles in immune function and cancer resistance. At the same time, a hormone called luteinizing hormone (LH) can spike to concentrations up to 30 times higher than normal and remain elevated for life. This hormonal disruption is linked to an increased risk of obesity, joint disease, urinary incontinence, thyroid problems, and certain cancers.

None of this means spaying is the wrong decision. It does mean that understanding the full picture allows you to take proactive steps to protect your dog's health after surgery.

Why This Conversation Matters More Than Ever

For decades, the standard recommendation from veterinarians across the United States was to spay all female dogs by six months of age. The reasoning was simple: it prevents overpopulation and reduces cancer risks.

But the science has evolved. In 2024, the World Small Animal Veterinary Association (WSAVA) published comprehensive new guidelines on reproduction control in dogs and cats. The 136-page document, published in the Journal of Small Animal Practice, concluded that routine gonadectomy may no longer be appropriate for all owned pets and recommended a more individualized, case-by-case approach. For large and giant breed dogs, the guidelines suggest that hormone-sparing alternatives like ovary-sparing spay should be offered as an option.

This represents a significant shift in thinking. The veterinary community is recognizing what the research has been revealing for over a decade: removing a dog's reproductive organs has health consequences that extend far beyond reproduction. And yet, many pet parents still walk into the exam room with no understanding of these trade-offs.

The Hormones Your Dog Loses After Spaying

A standard spay (ovariohysterectomy) removes both ovaries and the uterus. The ovaries are the primary production site for two critical hormones: estrogen and progesterone. Once they are gone, the body has no meaningful way to replace them.

Estrogen

Estrogen does far more than manage the heat cycle. In dogs, estrogen helps maintain bone density and skeletal integrity. It supports cardiovascular health. It regulates fat distribution and helps control appetite through interactions with leptin and other satiety hormones. It contributes to the health and resilience of cartilage and connective tissue. And it plays a role in urethral sphincter tone, which is why urinary incontinence is one of the most well-documented side effects of spaying.

Research published in the Journal of Animal Science confirmed that because spayed dogs lack estrogen, the appetite-suppressing effects of this hormone are reduced. The same study documented a measurable reduction in resting metabolic rate following the procedure, creating a dual pathway to weight gain: increased hunger paired with fewer calories burned at rest.

Progesterone

Progesterone is often overlooked in conversations about spay side effects, but its absence matters. Progesterone has documented anxiolytic (anti-anxiety) properties. It supports neurological function and influences mood stability. When progesterone disappears suddenly after surgery, some dogs experience increased nervousness, noise sensitivity, or general anxiety that did not exist before.

A 2019 study published in PLOS ONE examined the behavior of nearly 9,000 spayed female dogs using the Canine Behavioral Assessment and Research Questionnaire (C-BARQ). Researchers found that 23 behaviors differed between spayed and intact dogs, with spayed females showing increased rates of several unwanted behaviors, including fearfulness and certain types of aggression. The study concluded that the timing of spaying influenced behavioral outcomes, and that some dogs' tendency toward undesirable behaviors was affected by reduced lifetime exposure to gonadal hormones.

The Luteinizing Hormone (LH) Problem Nobody Talks About

Perhaps the most significant and least discussed side effect of spaying is what happens to luteinizing hormone after surgery.

In an intact dog, the pituitary gland releases LH, which signals the ovaries to produce estrogen and progesterone. The ovaries, in turn, send a feedback signal back to the brain to regulate LH production. This is a finely tuned loop. When the ovaries are removed, that feedback signal vanishes. The pituitary gland, no longer receiving the "stop" signal, begins producing LH at dramatically elevated levels.

Dr. Michelle Kutzler, a board-certified veterinary reproduction specialist and professor at Oregon State University, has been at the forefront of this research. According to Dr. Kutzler, LH concentrations in spayed and neutered dogs can reach up to 30 times higher than normal adult levels. Her 2020 review, published in the journal Animals (MDPI), documented LH receptors not only in reproductive tissue but also in the thyroid gland, adrenal glands, gastrointestinal tract, cranial cruciate ligament, urinary bladder, and lymphocytes.

The AKC Canine Health Foundation has funded ongoing research into this area. Their reporting summarizes the key findings: dogs spayed or neutered can have LH levels up to 30 times higher than normal, and this elevated LH may affect the thyroid, urinary tract, immune system, and even cancer development. Research in canine hemangiosarcoma and T-cell lymphoma cell lines has shown that LH receptor activation can stimulate cell proliferation, raising questions about whether chronically elevated LH contributes to higher cancer rates in spayed dogs.

This is still an emerging area of veterinary science, and more controlled clinical trials are needed. But the existing evidence is substantial enough that the AKC Canine Health Foundation, the WSAVA, and multiple university research teams are actively investigating the role of LH in post-spay health outcomes.

The Documented Side Effects of Spaying: What the Research Shows

Weight Gain and Metabolic Disruption

Weight gain is one of the most commonly observed side effects of spaying, and it is not simply a matter of overfeeding. A 2023 retrospective cohort study published in the Journal of the American Veterinary Medical Association (JAVMA) analyzed records from over 50,000 dogs across 900 U.S. veterinary clinics and confirmed a clear association between gonadectomy and increased risk of becoming overweight or obese. Data from the Morris Animal Foundation's Golden Retriever Lifetime Study found that dogs spayed or neutered at one year of age or younger faced twice the risk of becoming overweight or obese compared to intact dogs, while dogs altered after age one still had a 40% increased risk.

The mechanism is straightforward: estrogen helps suppress appetite and maintain metabolic rate. When it is removed, dogs burn fewer calories at rest while simultaneously experiencing increased hunger. Changes to metabolism and appetite can begin as early as three days post-surgery.

Joint Disease and Orthopedic Problems

The connection between spaying and joint disease is one of the most well-documented findings in modern veterinary research. A landmark 2013 study from UC Davis, led by Professors Benjamin and Lynette Hart, examined the veterinary records of 759 Golden Retrievers and found striking results. In early-spayed females (before 12 months), 8% were diagnosed with cranial cruciate ligament (CCL) tears compared to zero cases among intact females. Hip dysplasia rates were doubled in early-neutered males compared to intact males. Ten percent of early-neutered males developed lymphosarcoma, three times the rate of intact males.

The Harts expanded this research across 35 breeds in a 2020 follow-up published in Frontiers in Veterinary Science, and added six more breeds in a 2024 update. The pattern held: for many breeds, early spaying significantly increased the incidence of joint disorders including hip dysplasia, elbow dysplasia, and CCL tears. The researchers found that the absence of sex hormones during critical growth periods can alter the rate at which growth plates close, leading to abnormal skeletal development and increased vulnerability to orthopedic injury.

A 2016 UC Davis study on German Shepherd Dogs found that spaying or neutering before one year of age tripled the risk of one or more joint disorders, particularly CCL tears.

Urinary Incontinence

Urinary incontinence occurs in an estimated 5 to 20% of spayed female dogs, according to the 2024 WSAVA guidelines. This condition, sometimes called "spay incontinence," is directly related to the loss of estrogen, which helps maintain urethral sphincter tone. The AKC Canine Health Foundation's research found that spayed females with incontinence have more LH receptors in their urinary tract tissue, suggesting that chronically elevated LH may also play a role in this condition. Spay incontinence can develop months or years after surgery and is more common in larger breeds.

Behavioral and Emotional Changes

The behavioral effects of spaying are often underappreciated. While many pet parents expect their dog to become calmer after surgery, the reality is more complex. The sudden removal of estrogen and progesterone can affect mood regulation, stress tolerance, and social confidence.

The 2019 PLOS ONE study on nearly 9,000 female dogs found that spayed females showed increased rates of fearfulness, certain forms of aggression, and other unwanted behaviors compared to intact females. Five specific behaviors were associated with the age at which dogs were spayed, and twelve behaviors were linked to the percentage of lifetime exposure to gonadal hormones. Dogs spayed earlier, with less lifetime hormone exposure, tended to show more behavioral issues.

Increased Risk of Certain Cancers

While spaying eliminates the risk of ovarian cancer and significantly reduces mammary cancer risk (especially when performed early), research has identified increased risks for other types of cancer. The UC Davis studies found that late-spayed female Golden Retrievers had hemangiosarcoma rates four times higher than intact females. In Vizslas, spayed females were 4.3 times more likely to develop lymphoma than intact females. The AKC Canine Health Foundation notes that gonadectomized dogs show more LH receptor-positive lymphocytes, which may promote lymphoma development.

The cancer picture is complex and breed-specific. What is clear is that the relationship between spaying and cancer is not a simple "spaying prevents cancer" narrative. It prevents some cancers while potentially increasing the risk of others, and the balance depends heavily on breed, sex, and age at surgery.

Hypothyroidism

The prevalence of obesity and hypothyroidism increases after gonadectomy, according to the WSAVA guidelines. Dr. Kutzler's research identified LH receptors in thyroid tissue, raising the possibility that chronically elevated LH directly contributes to thyroid dysfunction in spayed dogs. Hypothyroidism slows metabolism further, compounds weight gain, and can cause lethargy, coat changes, and skin problems.

What You Can Do to Support Your Spayed Dog

Understanding these side effects is not meant to create fear or regret. Spaying remains a sound medical decision for many dogs, and the benefits of preventing pyometra and reproductive cancers are real. The goal is to be informed so you can take action.

Adjust Her Diet Immediately After Surgery

Metabolic changes can begin within days of spaying. Work with your veterinarian to reduce caloric intake by approximately 20 to 30% and shift toward a diet with higher protein and lower carbohydrate content to support lean muscle retention. Do not wait until weight gain is visible to make adjustments.

Maintain Consistent, Low-Impact Exercise

Regular physical activity helps counteract the metabolic slowdown that follows hormone loss. Swimming, leash walks, and moderate play sessions are ideal. Avoid repetitive high-impact activities, especially in the first year after surgery and particularly in large or giant breeds where joint vulnerability may be heightened.

Address the Hormonal Gap Directly

Diet and exercise are essential, but they cannot replace the hormones your dog has lost. This is where most standard post-spay care falls short. Joint supplements, multivitamins, and calming chews address symptoms, but none of them target the underlying endocrine disruption that drives so many of these side effects.

Hans (hansfordogs.com) was built specifically to fill this gap. It is the first daily chew designed to support the hormones dogs lose after spay, neuter, and natural aging. The product's core ingredient is velvet antler, a naturally occurring source of endocrine-active growth factors including IGF-1 and TGF-B. These growth factors support hormone signaling, metabolism, joint maintenance, and tissue repair by working with the body's existing biological pathways rather than introducing synthetic hormones.

What makes Hans different from generalized pet supplements is its specificity. It was formulated from the ground up to address the hormonal consequences of gonadectomy and aging. It is human-grade, clinically studied, and made from 100% natural ingredients. For spayed dogs showing signs of metabolic changes, low energy, joint stiffness, or coat quality decline, Hans targets the root of the problem rather than masking individual symptoms. You can learn more at hansfordogs.com.

Monitor Thyroid and Joint Health Over Time

Ask your veterinarian about baseline bloodwork after spaying, including a thyroid panel. Regular body condition scoring and joint assessments become more important in spayed dogs, particularly in breeds identified by the UC Davis research as being at higher risk. Early intervention is always more effective than reactive treatment.

Stay Informed About Emerging Research

The science of canine endocrinology is evolving rapidly. The WSAVA guidelines, the AKC Canine Health Foundation's ongoing LH research, and the work of researchers like Dr. Kutzler at Oregon State University are reshaping our understanding of what it means to remove a dog's reproductive hormones. Being a well-informed pet parent means staying current with this research and adjusting your care strategy as the evidence develops.

Spaying Is Still the Right Choice for Many Dogs

This article is not an argument against spaying. Pyometra is a genuine, life-threatening condition. Mammary cancer risk reduction is real and significant when spaying is performed at the appropriate age. Pet overpopulation remains a serious issue that responsible sterilization helps address.

The point is that spaying is a medical procedure with trade-offs, and pet parents deserve to know what those trade-offs are. When you walk out of the veterinary clinic after your dog's spay surgery, you should know that her hormonal landscape has fundamentally changed and that there are concrete steps you can take to support her health going forward.

Frequently Asked Questions

1. What are the most common hormonal side effects of spaying a dog?

The most common hormonal side effects include weight gain from a reduced metabolic rate and increased appetite, joint problems linked to the absence of estrogen's protective effects on cartilage and bone, urinary incontinence caused by the loss of estrogen-dependent urethral sphincter tone, behavioral changes such as increased anxiety or fearfulness from the sudden absence of progesterone, and a chronic spike in luteinizing hormone (LH) that may affect the thyroid, joints, immune system, and cancer risk.

2. How soon after spaying do hormonal changes begin?

Hormonal changes begin almost immediately. Estrogen and progesterone levels drop significantly within days of ovary removal. Research shows that changes to metabolism and appetite can start as early as three days post-surgery. LH levels begin rising within the first few weeks as the pituitary gland ramps up production in the absence of gonadal feedback. Most dogs reach their new hormonal baseline within two to six weeks, though the effects continue to compound over months and years.

3. Does spaying increase the risk of cancer in dogs?

The relationship between spaying and cancer is complex and breed-specific. Spaying eliminates ovarian cancer risk and reduces mammary cancer risk, especially when performed before the second heat cycle. However, UC Davis research across multiple breeds has found that spaying can increase the risk of other cancers, including hemangiosarcoma, lymphoma, and mast cell tumors, depending on the breed and age at surgery. The AKC Canine Health Foundation is funding ongoing research into how elevated LH levels after spaying may contribute to cancer development through LH receptor activation in tumor cells.

4. Can I do anything to replace the hormones my dog lost after spaying?

Standard supplements like glucosamine or fish oil support individual functions but do not address the root hormonal deficit. Products like Hans were developed specifically for this purpose. Hans uses velvet antler, which naturally contains endocrine-active growth factors like IGF-1 and TGF-B that support the body's hormonal signaling pathways. Unlike synthetic hormone replacement therapy (which is still in the early research stages and not widely available), Hans works with the body's existing systems to help restore some of the hormonal support that surgery removes. It is a practical, accessible option for pet parents looking to address the underlying endocrine gap.

5. Should I regret spaying my dog after learning about these side effects?

No. Spaying prevents life-threatening conditions like pyometra, which affects an estimated 25% of intact female dogs over their lifetime, and significantly reduces mammary cancer risk. The 2024 WSAVA guidelines still include spaying as a valid and important reproductive control option. The goal is not to create regret but to empower you with information. Understanding the hormonal consequences of spaying allows you to take informed, proactive steps, like adjusting diet, maintaining exercise, and considering targeted hormonal support, so your dog can live the healthiest, longest life possible after surgery.

Your Dog's Health Doesn't End at the Surgery Table

Spaying changes your dog's body in ways that go far beyond preventing pregnancy. The hormonal shift that follows surgery touches metabolism, joints, behavior, coat, energy, immune function, and long-term cancer risk. The research from UC Davis, Oregon State University, the AKC Canine Health Foundation, and the WSAVA all point to the same conclusion: these changes are real, they are significant, and they deserve attention.

The good news is that you are not powerless. Adjusting your dog's diet, keeping her active, monitoring her health markers, and bridging the hormonal gap with purpose-built support like Hans are all steps that can make a meaningful difference in how your dog ages after surgery.

Your dog gave you her trust when she went under anesthesia. Returning that trust means understanding what changed and taking action to help her thrive in the years ahead.