They can be very minor, to very severe. They could cause you migraines and other issues as well. Did you have any blood work? There are places where you can get the blood work done that is needed at a very reasonable price and ill be happy to suggest some for you. These are places where you bring the labs with you to a "quest" or a "labcorp" and they just draw the blood for you, you then put it in a box and pop it in the mail type thing.
As Bruce mentioned there are places to get the testing done for a reasonable price. One such company is shown in this link. The cost is $85 for the Free T3 and Free T4 tests along with TSH. That is what I would recommend because Free T3 and Free T4 are the biologically active thyroid hormones, and they reveal the most about your thyroid status.
http://www.healthcheckusa.com/Find-a-Blood-Test/46823/
Hypothyroidism can also affect a number of other areas as well. While thinking about your migraines, I recalled this info that I ran across recently.
"Migraine often occurs as a result of low brain levels of magnesium. One clinical trial found that supplementing with magnesium caused the complete elimination of migraine pain within 15 minutes in 80% of migraine sufferers."
So if you want to try and see if magnesium helps your migraines, I suggest 400 mg of magnesium glycinate daily, or similar, but not magnesium oxide, which is not as good to take. Also, it may sound crazy but I know of migraine sufferers that have stopped eating any chocolate or cheese, and their migraines disappeared.