Hi C.,
I'm not expert in this field, but here's what I know from a colleague's presentation.
You can't diagnose hypo- or hyperthyroidism on the basis of a single test, and you really need to know what test(s) were run. My guess is that the test result you're reporting if for TSH (thyroid stimulating hormone), in which case a level of 7.31 mU/L (miliunits/liter of blood) is an indication of *probable* hypothyroidism. You need results from a T3 and a free T4 test, however, to finalize the diagnosis. Normally in pregnancy, both of these would elevated above normal, nonpregnant levels. If you are truly hypothyroid, however, one or both of these will also return a low result. Your doctor should also test for antibodies to Thyroglobulin (TgAbs) and to Thyroid peroxidase (antiTPO), both of which may be present in hypothyroidism. The testing algorithm (method) is complex, so going to see an endocrinologist for a complete work up is a good idea. Also, no test is perfect, and there is always th possibility of false negative or false positive results. If it hasn't been done already, it might be worthwhile repeating the test to see if you get a different result.
Thyroid disease is far more common than most people realize, and 80% of those affected are women. A large number of cases are subclinical, that is there may or may not be symptoms, and only the TSH measurement may be off. The only way to really diagnose thyroid disease is by doing the blood tests. Also, thyroid disease can develop with time. It is entirely possible that you were NOT hypothyroid throughout the first trimester, and that your tests are showing a subtle shift in your physiology. After all, TSH values less than 7.0 mU/L are considered normal (although values between 5.1 mU/L and 7.0 mU/L would be considered borderline hypothyroid; between 0.1 mU/L and 0.29 mU/L would be considered borderline hyperthyroid, and and less tha 0.1 mU/L would be considered hyperthyroid).
Also, make sure you talk to your doctor about testing the baby for thyroid function after birth. This is done standardly, but depending on the results of your testing and possibly ultrasound, your child's pediatrician will want to know if your thyroid results had anything to do with proper thyroid development in your baby (this is pretty rare, though).
Good luck -- I hope all goes well with your pregnancy and that you deliver a beautiful, healthy child!