Women taking birth control pills release more of a substance called Thyroid Hormone Binding Globulin (THBG), which binds to thyroid hormones so that there is less to enable the body to function well (such as energy and immune system response). Oral contraceptives (OCs) also cause depletion of nutrients needed for healthy thyroid function and thyroid hormone production. Progesterone for example is shown to change mood, cause bloating and affect the thyroid by binding to the glucocorticoid receptor.  This is the same receptor which cortisol utilises – stimulating our flight or fight response when we are stressed. Prolonged periods of stress has been shown to lower the immune system, a key component of our ability to fight off infections such as UTIs.