18 month female--no obvious heat

Question
I have an 18 month old female boxer we purchased in France.  She is our third boxer baby.  We are concerned that we have not noticed her in heat yet, i.e. noticed any bleeding.  Additionally, we do have an unaltered male in the house, so I think he would let us know if she was in heat.  Any thoughts?  Thanks.

Answer
Well since you have owned  Boxers before, you are familiar with their heat cycles and the symptoms.

18 months old is awfully old to not have had one yet.

The main cause could be a thyroid disorder. It stops heats, prevents heat cycles, prevents pregnancies, or causes miscarriages too.

Is she a little thin or over weight, shed more than the others, eat fine but doesn't gain weight, sleepier than the others, any differences?

I would suggest you take her to your vet for a blood test for thyroid. They draw blood from the artery in the neck. If she has thyroid problems, ask your vet for Pala-Tech chewables. They love those and they are the least expensive of all thyroid meds.