Aw, nuts.

If you’ve noticed your balls have gotten smaller, it could be a sign of an underlying health condition.

Before jumping to get scrotal filler to plump things up, it’s best to get to the bottom of what’s caused your family jewels to depreciate in the first place.

Dr. Alexander Pastuszak, a urologist and male reproductive medicine specialist, spoke to The Post about six issues that could be at play — including a painful condition that can require testical removal and an inevitable change that all men can look forward to.

#1: Meds that slow T production

External testosterone is “probably the most common cause of acquired shrinkage,” Pastuszak said.

As many as 11 million men are on testosterone replacement therapy, or TRT. When the body struggles to make enough testosterone on its own, it introduces a host or problems, from sex performance, to trouble losing weight, and fatigue.

TRT is a common solution to this issue, but it presents another one: Because the meds add enough testosterone, the body stops making any more.

Then, all the tubes that are no longer being used as often to release hormones “shut down and shrink,” Pastuszak said. Testicles may noticeably wither “within a few months of consistent use.”

With TRT, shrinkage is expected and not necessarily a problem. Men on TRT may see shrinkage but all the benefits of the medicine at the same time.

Testes shrinkage also happens with steroids, which are synthetic or modified testosterone. Sometimes it’s prescribed medically, like TRT, but other times it’s misused to enhance performance in sports or bodybuilding.

The effects can be more dramatic as steroids are often taken at a much higher dosage.

#2: Twisted testicles

At the base of the balls, there’s a cord of vessels, nerves, and tubes that hold the testes inside the skin and muscle of the scrotum. If that cord gets injured — through with injury, heavy exercise or sometimes without a clear reason at all — it can cause serious problems.

Called testicular torsion, this condition is defined by blood supply cut off to the testicles, which can cause permanent damage. If blood supply is cut off for an extended time, the testicles may need to be surgically removed.

It’s an emergency that often requires surgery. If not treated within six hours, the testicles may lose its ability to function. In some cases, function is lost in even less time. Even if the torsion is corrected, shrinkage could happen days or months later.

#3: Enlarged veins

Varicocele are enlarged veins inside the skin that holds the testicles. These can develop over months or years, says Pastuszak.

Each testicle has its own artery to transport oxygenated blood in, and its own main vein to transport depleted blood out. Smaller veins connect to the main one to transport depleted blood, which can back up and swell, becoming varicocele.

This happens more often on the left testicle, which follows a slightly different path than the right.

The disrupted blood flow makes it harder for the testicles to regulate temperature, making it hotter.

The raised temperature and pressure from the bulging vein can stunt or shrink the testicle, Pastuszak said. It can also cause infertility: About 10-20% of men with varicocele have trouble fathering a child.

#4: Infection and swelling

A bacterial or viral infection can cause swelling and inflammation in one or both testicles, sometimes causing a complication where the testicles shrink.

Mumps are the most common cause of viral orchitis, or testicle swelling. Nearly a third of men who get mumps after puberty get orchitis, around 4 to 7 days after the mumps start.

The main reason people get mumps is because they weren’t vaccinated. Since the vaccination program started in 1967, mump cases have decreased 99% in the US.

Orchitis can also be caused by a bacterial infection, often of the urethra or bladder that spreads to the epididymis, located at the back of the testicle. Less often, an STI can lead to orchitis.

#5: Hypogonadism

Low testicular functioning (despite getting the right signals from the brain) is called primary hypogonadism.

Klinefelter syndrome is the most common genetic cause of this. This affects around 1 in 600 men and is treatable with TRT. “The testicular tissue itself fails, so testes are firm and small,” Pastuszak said.

Cancer treatments can also cause primary hypogonadism.

And don’t forget: Normal aging

When men get older, their bodies produce less testosterone and less sperm. There’s also less blood flow.

This results in less volume, or natural shrinkage. This usually happens after age 60 and continues to age 90.

When to see a doctor

Testicles that have always been small may reflect a longstanding condition, while shrinkage of a previously normal testicle is a “more concerning red flag,” Pastuszak said.

See a doctor if you’re seeing reduced function, asymmetry, or a lump — ideally a urologist or andrologist.

“Go promptly if shrinkage comes with sudden pain, swelling, a lump, or other new symptoms,” he said. “A scrotal exam, scrotal ultrasound, and bloodwork (testosterone, LH, FSH, sometimes a semen analysis) can sort out the cause.”