MayoClinic.com has a comprehensive resource page on hypothyroidism.
EndocrineWeb.com features information on a number of thyroid conditions, including both hypo- and hyperthyroidism.
The American Thyroid Association features a comprehensive online booklet outlining thyroid disorders and steps those suffering from such conditions can take to cope.
The National Institutes of Health offers a comprehensive database of information and links to additional resources on thyroid conditions.
The Hormone Foundation provides a number of overviews specific to thyroid conditions, as well as guides to help pregnant women dealing with thyroid disease.