what would cause tsh levels to drop
Such low levels are not always necessary, but decreased T4 and TSH levels are an indication that T4 and RT3 levels are being well depleted (which is often the point of T3 therapy). Hyperthyroidism often causes very low TSH levels and symptoms of hyperthyroidism. Yet proper T3 therapy often causes very low TSH levels without symptoms of hyperthyroidism while providing normal temperatures and a decidedly euthyroid clinical state.