The self-regulation mechanism, and how to use it in our benefits

If the body naturally and continuously produces sex hormones, why don’t we end up with too much of them? Well, obviously because the body is smart and able to self-regulate the production. And understanding the way it does that can help us understand and choose better the ways of our medical transitions (at least for transfeminine people).

The production of sex hormones is the final step of a chain of signals and reactions that starts in the brain. A part of the brain (the hypothalamus) releases in a pulsatile way (i.e. in a regular temporal pattern) a hormone called GnRH (Gonadotropin releasing hormone). GnRH is received by a gland in the lower part of the brain - the pituitary gland (or hypophysis), which, in response, releases two hormones called gonadotropins: FSH (Follicle Stimulating Hormone) and LH (Luteinizing hormone). Those two will go down to the gonads (ovaries and testes) which, in response, will produce testosterone (for the testes), and estradiol and progesterone (for the ovaries).

This sex hormone production will continue, and the levels of estradiol/progesterone/testosterone will rise, and circulate in the body. They will be received by the brain, and after reaching a threshold, eventually send the signal to both hypothalamus and hypophysis to stop the production of both GnRH, and FSH/LH. In other words, when the sex hormones reach a level that the body judges to be correct, they turn into a messenger ordering to stop their own production. This part of the circle is what we call negative feedback.

It should be noted that this mechanism is a bit more complex for AFAB people with a full reproductive system, as the regulation of hormones must allow for the cycle to work with the different phases (stimulation, ovulation, menstruation…). We will not detail this system here as this is not a website about reproductive health.

Now, what does this mean for us? It means that we have a way to control the production of our sex hormones - or at least to stop them.

• On the one hand, the brain will respond indifferently to all androgens, estrogens and progestogens to activate the negative feedback. For the brain, they are all read as sex hormones.
• On the other hand, the gonads can produce only one type of hormone, the one type whose production will be stopped by the negative feedback mechanism. That means that by adding enough of any sex hormone in the bloodstream from the outside (this is called exogenous - from an outside source - as opposed to endogenous - body made), and even if it is not the hormone that your body produces naturally, you can stop the production of the only hormone your body can produce.

So, in the case of feminizing HRT, we add exogenous estradiol; if the levels are high enough, the brain will receive the signal “ok, enough sex hormones” and send the signal to the gonads: “stop the production”. If you’re AMAB, with testes, the only thing your body can do is to stop the production of testosterone. If you keep this level, you end up with the ideal hormonal balance for feminization. Your estradiol will be medium-high to high, your testosterone will be low, and so will be your LH/FSH. Inversely, for AFAB people, taking testosterone will decrease your LH/FSH, and as a consequence, stop or significantly reduce your production of estradiol.