Lack of vitamin D contributes to the possibility of low thyroid. Also, it means that many irritants are likely to aggravate your thyroid to a greater extent if you lack vitamin D. For example, many chemical irritants in the environment irritate your thyroid gland, such as perchlorate and fluoride. If you lack vitamin D you are more likely to be adversely affected by them.
This may be part of the reason that so many people feel metabolically worse and gain weight as the winter months move along. It is simple to make sure you take some extra vitamin D in the winter. Doing so may help you keep your metabolism and thyroid from suffering the “winter blues.”