You should read up on iodine. Iodine receptors are found in every part of the body, including the reproductive organs, not just the thyroid. Painful periods, heavy periods with clots are a sign of iodine deficiency. Do you have FBD (fibrocystic breast disease)? That is another sign of iodine deficiency. Some links for more information:

http://www.breastcancerchoices.org/iodineindex.html
http://www.iodine4health.com
http://health.groups.yahoo.com/group/iodine/
http://www.optimox.com/pics/Iodine/opt_Iodoral.htm
http://www.optimox.com/pics/Iodine/opt_Research_I.shtml
Dr. David Brownstein "Iodine, Why you need it, Why you can't live without it" (amazon has it)

When you are deficient in iodine, other lighter halides like bromide and flouride can attach themselves to the iodine receptors causing many health issues. Detoxing out the bromides and becoming iodinated will help restore balance to your hormones and make your periods lighter and easier.