Right, but it's not so much 'suppressing' as evening out the peaks and the troughs. You don't want (comparatively) very high or very low (nothing) levels. Which is what is happening with your own levels during peri. If you take HRT, your lows are not so low and your peaks comparatively not so high (because the lows are not as low) - so you even everything out. So it's not really 'suppressing' your own levels, it is just adding in something constant.