Neutering a Cat: 20-30% Lower Metabolism, 20% Gain

Neutering removes the main source of sex steroids, and the body adjusts within weeks: resting energy use falls 20 to 30%, appetite rises 20 to 25%, and a cat fed the same bowl gains about 20% of its body weight by 6 months. None of that is a disease. It is a predictable shift in the energy budget, and it is manageable with 2 numbers — calories per day and weight per month.

What the surgery actually removes

Castration removes testicular testosterone; ovariohysterectomy or ovariectomy removes estradiol and progesterone. Both hormones do more than reproduction: they suppress appetite at the hypothalamus, raise basal metabolic rate slightly, and hold back fat storage. Remove them and the negative feedback on LH and FSH disappears, so those rise within days, but the metabolic effect is what owners notice first — a cat that stops yowling, stops roaming, and starts asking for food at 05:00.

Resting metabolism drops 20 to 30%

Resting energy requirement is estimated as 70 × body weight in kg to the power 0.75. For a 4 kg cat that is about 198 kcal. An intact adult is usually fed at 1.3 to 1.4 times that, roughly 257 to 277 kcal. A neutered adult needs 1.1 to 1.2 times, roughly 218 to 238 kcal. That is a gap of 40 to 60 kcal a day, which sounds small until you multiply it: 50 kcal a day is about 18,000 kcal a year, close to 2.3 kg of fat if nothing else changed. Fat is not storage alone: it releases hormones of its own, which adipose tissue as an endocrine organ covers.

Measure	Before neutering	6 months after	Target
Resting energy, 4 kg cat	198 kcal	198 kcal	Unchanged
Daily calories fed	257 to 277	218 to 238	1.1 to 1.2 x RER
Dry food at 3.9 kcal/g	66 to 71 g	56 to 61 g	Weigh, do not scoop
Body weight	4.0 kg	4.8 kg	4.0 to 4.4 kg
Body condition score	5 of 9	6 of 9	5 of 9

Appetite climbs 20 to 25%

The increase is not greed. With estradiol gone, the satiety signal from the hypothalamus weakens, and many cats eat 20 to 25% more per sitting while also sleeping 1 to 2 hours longer. Free-feeding a neutered cat is the single most common route to obesity: a bowl refilled at 08:00 and 20:00 lets a cat take 90 to 110 g of dry food a day, which at 3.9 kcal/g is 350 to 430 kcal, nearly double the 218 to 238 kcal it needs. Measure the day's ration once in the morning and put the bowl away.

The 20% at 6 months is a trend, not a fate

Weight gain clusters in the first 6 months and then flattens if intake is matched. Left free-fed, a 4 kg cat reaches 4.8 kg by month 6 and 5.3 kg by month 18. The fix is boring and works: weigh the cat every 2 weeks for 6 months, write it down, and cut the daily ration by 10% at the first 200 g rise. A cat already overweight should lose 1 to 2% of body weight per week, which for 5.5 kg means 55 to 110 g a week, no faster, because a cat that stops eating for 2 to 3 days can develop hepatic lipidosis.

Hypothyroidism is rare; hyperthyroidism is not

Naturally occurring hypothyroidism is genuinely rare in cats, and most cases on a chart are iatrogenic: a cat treated with radioiodine or a thyroidectomy for hyperthyroidism overshoots into low thyroid. The opposite problem is common — roughly 1 in 10 cats over 10 years runs high, which shows as weight loss with a good appetite and a heart rate of 240. That is why a thin older cat with a ravenous appetite needs a T4, not a diet. Thyroid hormone also sets the metabolic rate, so a cat on treatment for hyperthyroidism often gains 0.5 to 1.0 kg in the first 3 months as the rate normalizes.

Timing changes growth, not just fat

Sex steroids close the growth plates. Neuter at 8 to 16 weeks and the long bones keep growing 2 to 4 weeks longer, which is why pediatric neuters often end up 5 to 10% taller at the shoulder with a slightly narrower chest. Neuter after 6 months and skeletal shape is essentially set. Neither timing is wrong; the difference is that early neuters need their calorie ceiling set at 4 months, before the appetite rise arrives. Frame size differs enough between breeds that the same 20% looks very different on a 3 kg and a 7 kg body, which is what the breed quiz is for.

A plan that survives the first year

Switch to the neutered-adult calorie target on the day of surgery, not at the 6-month check. Feed 2 measured meals and, if the cat screams, split the same total into 4. Weigh every 2 weeks, score body condition every 4 weeks, and use a kitchen scale for the food, since a scoop varies by 15 to 20%. Keep 10 minutes of chase-the-string play twice a day; that alone is 20 to 30 kcal. And book the 6-month weigh-in even if the cat looks fine, because the eye is a poor scale at 10% body fat. Owners who weigh food and cat on the same kitchen scale tend to catch the rise at 200 g, when a 10% cut in the bowl fixes it in 4 weeks; owners who judge by eye usually catch it at 1.5 kg, when the plan needs 6 months and a calorie count.

When to call the vet

Call for weight gain of more than 15% in 6 months despite measured feeding, a body condition score of 7 of 9 or higher, drinking and urinating more than usual over 2 weeks (a diabetes screen), any weight loss over 10% without a diet change, vomiting more than twice a week, or a cat that refuses food for 48 hours. Obesity is the risk factor behind diabetes and joint disease in this species, and both are easier to prevent at 4.8 kg than to treat at 7 kg.

Terms mentioned in this article

Frequently asked

How much weight will my cat gain after neutering?

About 20% in 6 months if the bowl stays the same, so a 4 kg cat reaches 4.8 kg. The gain comes from a 20 to 30% drop in resting energy use plus a 20 to 25% rise in appetite, not from laziness. Measure the daily ration and cut it by 10% at the first 200 g rise, and most cats stay near their surgery weight.

Does neutering cause hypothyroidism in cats?

Rarely, and almost never on its own. Most low-thyroid cats in practice became that way after treatment for hyperthyroidism, such as radioiodine or a thyroidectomy. The common thyroid problem in cats is the opposite: roughly 1 in 10 over 10 years runs high, with weight loss, a good appetite, and a resting rate near 240.

Should I feed less right after the surgery?

Yes, from the day of surgery. A neutered adult needs 1.1 to 1.2 times resting energy instead of 1.3 to 1.4, which for a 4 kg cat is 218 to 238 kcal a day rather than 257 to 277. Keep the same number of meals, weigh the food on a kitchen scale, and reweigh the cat every 2 weeks for the first 6 months.