It's very common, especially after 2nd babies, in women who have family history on the mother's side or who have had borderline thyroid levels all along (not that you would have known that since it is not routinely checked).
Postpartum, your progesterone levels plummet from being very very high to rather low. This lack of progesterone, or rather, the change in progesterone inhibits the conversion of T4 hormone to the active T3 hormone. Because it affects the conversion of T4 to T3 and not necessarily the production of T4, you end up with Hashimoto's Thyroiditis, which is tricky to balance. Sometimes hyper, sometimes hypo. Very frustrating.
Selenium is helpful in conversion of T4 to T3. Thyroiditis is technically an autoimmune issue, so stabilizing the immune system is important, too. have your vitamin D levels checked whenever there is a suspicion of autoimmune problems or thyroid problems. Nearly always, Vitamin D levels are low.
Aimee wife to Matt Mama to Asher (4) and Ari (Due 11/6/10)