99% of circulating thyroid hormones are bound to carrier proteins, which renders them metabolically inactive. The remaining “free” thyroid hormone (the majority of which is T3), binds to, and activates, thyroid hormone receptors, causing biological activity. Very small changes in the amount of carrier proteins will affect the percentage of unbound hormones.
Oral contraceptives
Pregnancy
Conventional female hormone replacement therapy all the above may increase thyroid carrier protein levels and, thereby, lower the amount of free thyroid hormone available.