Transit time just means the amount of time it takes for food to get from one end out the other. There are a few possibilities that might account for her slow transit time...and it is also possible that there is a combination of factors. The doctor says her thyroid is okay, but her metabolism is slow...but the thyroid is responsible for metabolism...so I'm not sure what is meant here. The blood tests for thyroid functioning are not always an accurate reflection of what is happening with her thyroid hormones...she may be manufacturing them fine, but her body may be creating antibodies to destroy them...known as Hashimoto's disease. You might want to do a bit of research on basal temperature testing for thyroid functioning and start keeping track of her temps. Any problem with her thyroid functioning/metabolism could effect her bowel functioning and result in increased transit time.
Weak peristalsis can also result in increased transit time...the muscle tone is just not strong enough in the bowel to efficiently move waste through the intestines. Lack of adequate fiber, poor general muscle tone, some neurological conditions and overuse of laxatives (including laxative herbs) can contribute to impaired peristalsis.
Another possibilty is that she is simply so impacted that even though she goes fairly often...she is just incredibly backed up and it is causing a blockage. There are even people who move their bowels a couple of times a day and may still be constipated...because it is not only the frequency of movements, but the quantity that is eliminated. The movements should be complete and result in a satisfied empty feeling...passing a pellet or two is not going to clear out the waste.
And...again...it is possible that she is having a reaction to something she is eating or something in her environment. Allergies, sensititvities and reactions...whether to chemicals, foods, inhalants, etc....can have a great effect on digestive functioning...including constipation, gas, bloating, weight gain...as well as creating a variety of symptoms in any system of the body. The presence of any pathogenic organisms (ie. candida, parasites) does likewise and should be ruled out.
I hope this is helpful...I will be away from my computer for the next few days while I'm moving. If you have any additional questions for me...please hold off for a few days and post them in a new thread and I will get back to you ASAP! ; )