Causes of
estrogen dominance include birth control pills,
estrogen replacement therapy, stress, poor
diet, obesity, xenoestrogens (excess estrogens found in anything from pesticides to plastics).
Estrogen dominance can affect conversion of T4 to T3 as well as increase thyroxine binding globulin (if there is too much thyroid binding globulin, thyroid hormone is bound and unable to get into the cells).