You can avoid the downsides by being very informed. Sounds like you're still early in the journey.

I sympathize with being 3 sigma on the distribution... I'm four sigma out myself. My total T is around 1200ng/dl with most everything in my bloodwork related to hormones being a genetic freak. And, yes, I'm natural. Even with my extremely high T - probably twice yours - I struggle to gain any muscle at all and look like I've never lifted a weight in my life.

I'm still doing steroids next year because I'm tired of not seeing results for my insane effort.

If you're just growing in the stomach then you're doing something wrong. Check macros - should be overly high protein. Eat only 500 calories surplus per day. Your stomach will get larger just from food volume. Your stomach will always be large if you're on a bulk - it's supposed to be full all the time.

After you gain 20lb over about 20 weeks - reasses. As it is - it sounds like you haven't done really serious tracking. When I'm bulking/cutting - I don't eat anything unless every ingredient goes on a scale and I track everything down to the gram. One way to make less lifestyle less painful is to do meal prep and label all your meal tins with the macros. (Or just be obnoxious and measure everything out where each tin is the exact same - harder to do)

Get a dexascan a couple weeks into your bulk then a couple months in. You'll be able to tell if you're only gaining fat or if you're gaining lean body mass. Same process with cutting. Your eyes might not be well trained.

That is a fantastic insight, and a wild natural T level, especially with the low muscle gain! (As you inferred I measured at 650ng/dL with sample size 1; not sure how it flows with time; didn't get a free-test measurement, but should do that; kind of curious how that plays into your situation too) I was expecting to measure low based on the muscle thing + generally low-superficial-T features like minimal facial hair, and low body hair/thin eyebrows, then would have gone straight to it, but... I paused to reconsider after the measurement.

Doctors don't take stuff like this seriously (Varies wildly, but I have a number of family/friends who are doctors, and was in a military field under flight doc supervision/physicals for most of my career). I imagine it's not on their list of medically significant conditions as it's a superficial one. They deal with things that cause immediate or future health problems (or med disqualification for the job), or effect quality-of-life directly. Being extremely skinny (High or normal test for us respectively!) doesn't register as it's superficial/aesthetic. I wonder if your hormone-related identification would be more actionable as you have that number? While 6'2 / 130lbs (Not sure your example) is extremely rare, without any direct consequences on health, it doesn't register. Nothing to fail me on a physical for, no treatment plans that fit, and doesn't pattern match with people checking into hospital.

I think re growing the stomach - I meant more that responds very quickly to changes in diet in either direction; I can go from pudgy/belly to flat abs in a few weeks. I'm hoping the ab work will help a bit with overall look in well-fitting clothes or shirtless, and help with rib flare, but it's probably skippable compared to the bigger lifts.

For the macros, you're right: I have not being doing any tracking. I have been (For the past 6 months when I did a big shift) eating slowly, and paying attention to hunger/satiation signals. No longer finishing a meal just because it tastes good, if I'm no longer hungry. I have cut out all carbs-sans-fiber. No sugary foods (other than fruit, dark chocolate etc), no refined grains unless eating out. Lots of veggies, meat, eggs, and fruit + whey shakes. So, high protein, high fiber, normalish carbs and fat. As you say, probably need to load on the protein more. (More meat/whey/eggs/peanut butter?) How has the tracking worked out for you; any insights? I'm curious about the effort involved, and accuracy of any numbers put in.

I should do the dex! I'm curious; just looked up; $130 near me; will do it! The 12% estimate was based on visual cues like bicep vein, muscle definition, appearance of facial bones/masceters etc. If I eat lazily, it goes up to probably 18-20%; then I look like I have never seen a gym. Would love to see the actual number.

What is stopping you from doing the steroids now? Test or a mix? Worried about the hairline / body hair or negative face shape changes? I am giving this one final push, then will probably hop in in 1-2 years if this still isn't working. My lifts have stayed the same or gone slightly up while I've been getting lean. The lean thing for is a realization that I am accepting the build, and don't want to hide the muscles I have. I love the way I look without a shirt or in tight-fitting clothes now, but still have an extreme BMI.

Btw, that last point seems to be also one of those counter-intuitive things: For the ultra-skinny (Or maybe more acurrately for anyone who's lean, but has some muscles), looking good without a shirt or in form-fitting clothes is a completely different beast from not. Can go from "Looks fit" to "Doesn't know what a gym" is from a change of clothes alone!