My dog has one testicle that is almost twice the size of the other. This is something we’ve just noticed today. He

Okay, thanks for that information. Is it extremely important to you to use him for breeding? Is there any possible exposure to people in the house who use topical hormone replacement therapy?

So when it comes to abnormal testicular size,

• Where one is too small in a young dog, previous testicular torsion can be of concern. If the testicle isn't painful then monitoring for further changes is fine. Other hormonal problems such as adrenal gland problems and thyroid problems can cause this too but these aren't very common in a one year old dog. This can cause infertility for some dogs, due to any cause but hormonal problems may be treatable. Some blood tests and an ultrasound are the best first diagnostics for this. 
• If one is too large, then blood tests and ultrasound can be helpful but neuter and biopsy are the most definitive diagnostics. A too large testicle can be due to certain types of infections (some are transmissible during mating) or due to tumors (some are benign though some may not be, so this is where biopsy is of particular importance). In these cases, breeding is not recommended and even discouraged. 

Because sometimes it can be difficult to tell for sure if one is too small or one is too large, it may be worth confirming. There are measurements that can be done to help assess circumference in relation to body mass, so we know for sure that he's not at risk of future health conditions.

Does that all make sense?

Okay, great. Do you have any other questions or concerns that I can be sure to address for you?

That sounds like a great plan. If you find you develop any other questions or concerns please feel free to send another message here on this chat. I’ll be more than happy to help with any follow-up queries you may have. If I’m not online when follow-up messages arrive, I’ll be sure to reply as soon as I log in again! Thank you for using JustAnswer.

Dr. Nicole