Heyia there. I know this topic is old, but if you're still out there, or if anyone else experiencing this is looking at this topic, as I am, here's a reply.
I'm Phoenix. I'm 21 years old. Like you, I have thyroid agenesis, and I have taken medication all of my life. Also like you, I've taken the initiative to understand my condition and my body, and as a result, I know more about congenital hypothyroidism than many endocrinologists do.
Going without your medication, first and foremost, is never an option. Thyroid hormones are absolutely necessary, and the death that you would die would be slow and painful at this stage of life. You would not think to rip your thyroid gland out (if you had one) and not taking your medication is the equivalent of removing your "thyroid gland" in this case. I've been there, done that, and it's miserable.
Please remember that doctors don't know anything more than what they're taught, and much of what they're taught is that we, as patients, don't know as much as they do, and thus don't know what we are talking about. That can be true, although I have done my best to educate myself about my body, my condition, and appropriate treatment over the past half a decade. But even the least educated patient knows how they feel better than a doctor ever could, and doctors are often unwilling to acknowledge that feeling like crap, even if your levels are "within range," means something is still wrong.
If you can get a sense of how you feel when your levels are off, as I have, you'll be able to predict the results of many blood tests, and even help your doctor treat you more effectively. I always know when my levels are high or low, and by about how much. I also know more about medication dosing than my doctors do. My best doctors have generally asked me what I think needs doing, because I know my body and my illness so well.
At 26, there is no way that you should be taking less than 200 mcg of medication, give or take depending on the type of medication. I'm not hugely familiar with Armour's dosages, but I do know that I was below the 100 mcg range before double digits, in the 100 mcg range during adolescence, and in the low-200 mcg range since I was about 17 or 18. The amount you need continues to increase as you're developing, and will change throughout your life, just as your thyroid gland would adjust how much hormone it produced throughout your life.
Your life-span, believe it or not, is perfectly normal with proper care and diligence. Take your medication EVERY MORNING, see your doctor every 6-12 months (or more frequently, as needed) and get your blood tested. See a specialist if your system is "testy" like mine is - I'm very sensitive to changes in my medication, so a primary care doctor is not enough. Find what works for you.
As for children, you can carry to term, so long as our doctor keeps a very close eye on your thyroid levels. Too low of levels can cause birth defects and miscarriage. Too high of levels will only affect you, as the placenta will protect the baby from getting any more thyroid hormone than it needs, but it still can cause miscarriage if your body is under too much stress from the high level.
Stress can definitely affect your levels, and as is clear, low thyroid levels cause extreme depression and issues with skin, hair and nails. Other things can include products that interfere with absorption - iron, calcium, multivitamins, soy products. Avoid these for 3-4 hours before and after taking your thyroid medication, which should, ideally, be taken in the morning, as soon as you wake up. I put mine in a pill box on my bathroom counter, so that when I wake up and use the bathroom, I see the pill box and remember to take my medication.
If you'd like to talk to someone else who is living with this and share experiences, tips and tricks, or whatever else, feel free to message me.
Phoenix