Your body would only stop producing it yourself if you were taking a large enough dose to fulfill all your bodies requirements and keep your tsh suppressed to near zero - this dose is different for everyone but might be 150-200mcg give or take so 25mcg will not do this, it will just add to your own production. If you have been diagnosed with hypothyroidism, then your body is not making enough and you will need to add to it to bring down your tsh.
The tsh is not the level of thyroid hormone in your blood, it is a hormone the body makes to request more thyroid hormones to be produced when it needs them. If this hormone is being listened to and your thyroid is able to respond to this request, your tsh levels should never really be over a certain range. Once your tsh is too high, it means it's basically shouting at your thyroid to make more thyroid hormones but for whatever reason, your thyroid cannot so the tsh gets higher and higher which is basically screaming louder and louder at thyroid for more thyroid hormone.
Not sure if explained properly enough but think of the tsh as being the screams of a newborn baby when it wants feeding - they might mumble a bit at first, make a few whines and so you know they're getting hungry so give them milk straight away which soon shuts them up. The longer you leave it, the more they cry and the louder they cry until they're red in the face because you are not responding to their needs lol!
I've never heard of anything in regards to a problem if you take less thyroid hormone than you normally produce so must be some confusion there? If you are only naturally producing 50% of what you need, then you would need to replace the rest to function properly. However, some who are autoimmune will prefer to take enough meds to suppress their tsh as it can possibly help lower the level of attacks on the thyroid from thyroid antibodies but also especially if your own production is a little sporadic and not consistent enough. Bodies like their hormones to be stable.
It takes a while to build up your dose as it takes six weeks for the effects of that dose to show itself in the way you feel and blood test results to see if you need more or not so you keep doing it gradually until you get to the right dose. Plus because your body is used to working on not enough thyroid (which will slow your metabolism down etc to cope with this), it would be a bit of a shock to suddenly give it full dose. Your body needs time to adjust back to full working speed.
Hope that makes a little more sense lol