If they only measured your TSH and not Free T3 and Free T4 and antibodies, you cannot be sure it is not a thyroid issue. Also, there are new lab ranges for diagnosing hypo (TSH > 3.0), however few of the labs are actually using the new range. So, unless your doc is very knowledgable about thyroid they could miss this. You could also be borderline (i.e., symptoms are starting before the labs really show whats going on AND the labs are meaningless if you have antibodies).
However, since you have no other symptoms, this is likely due to a nutritional defiency. You don't mention whether you breastfed or not, but nature ensures that the baby gets all the necessary nutrients at the expense of the mother. Thus, nutritional deficencies are not at all uncommon. Hair is the least important thing, so your body will conserve the nutreints for more vital processes. You might ask your doctor (family doctors are usually far more knowledgable in this are) to check your vitamin B status, include B6, B12 and folate. In particular Biotin is important for hair (and nails). Also, vitamin E and zinc. And, also checking iron levels (i.e., ferritin) is not a typical lab run by most docs. So, you should get a copy of your labs and make sure your doctor ordered that.
Also, seeing a nutritionist to evaluate your diet might also be a good idea. A good one can tell you what foods to eat that will help. In addition, whether taking supplements will help. Few conventionally trained MDs know alot about nutrition, since they usually get one course.