Birth control does not cause low TSH levels. The count of free thyroid hormones remains normal in women on the pill. Thus, the level of the TSH, the pituitary hormone responsible for the creation of thyroid hormones, also remains normal, according to the Thyroid Foundation of Canada.
Elevated estrogen levels, sometimes resulting from the pill, cause an increase in the protein that binds thyroid hormones. To compensate, the body increases its production of binding thyroid hormones in order to maintain a normal level of nonbinding thyroid hormones. It is only these free hormones that are believed to function at the tissue level, and the body ensures the normalcy of their measurement. Therefore, TSH levels are maintained in circumstances where thyroid hormonal counts change due to birth control, as explained by the Thyroid Foundation of Canada.