Welcome to ATD. Thank you for the query.
Thyroxine does not affect the combination pill action but the combined pills do affect thyroxine action.
Estrogens tend to increase serum thyroxine-binding globulin (TBg). In a patient with a nonfunctioning thyroid gland who is receiving thyroid replacement therapy, free levothyroxine may be decreased when estrogens are started thus increasing thyroid requirements. Therefore, patients without a functioning thyroid gland who are on thyroid replacement therapy may need to increase their thyroid dose if estrogens or estrogen-containing oral contraceptives are given.
So just let your doctor know that you are taking combination pills, so that he may adjust the thyroxine dose accordingly.
Hope this helps. Take care
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