You have a doctor? Because most of doctors don't put guys on TRT if testosterone levels are in 400s.
At 21.. it's a tough decision for me to recommend going straight to TRT.
I would first try bulking. Maintain a caloric surplus for many months and stay true to it. If you aren't gaining 1/2 a lb or more per week, you are not in a surplus. If you are maintaining a surplus with shitty food, expect shitty blood levels.. you cannot expect to have ideal output, without ideal input. Whole, healthy foods. Limit processed garbage, pop, high sodium, simple sugars, etc. After you have consistently and healthily gained weight every week for 3-6 months, then have bloodwork done (at both 3 months, and 6 months) and see how well your body is working for you then.
Stimulate the body naturally with a training regimen that will challenge you, but being the nature of your current problem take more rest than a normal person would. Try 1 day on, 1 day off to begin with. Do light cardio on 3 off days to improve cardiovascular health and overall health. Gauge your rest needs by your progress. If you aren't seeing increases in strength, or size.. it's likely because you've broken down the muscle again, or wore out the CNS before it had time to recover. If you truly have low T, you will need to accommodate that the entire time it remains that way. If this helps, and your levels rise after a few months.. you may be able to workout more frequently or more intensely.
You may just be over worked all the time right now.. stressful life, stressful or physical/repetitive job, frequent training, bad food, infrequent rest are all going to add up to a powerful cocktail poised to destroy your hormonal system! Identify problems, research solutions, create a plan, initiate plan. Test results.
ALWAYS sleep 7-9 hrs. No excuses. Tell off your friends or family unless it's an emergency.. Commit.
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After you've been under a protocol like this for 12-24 weeks.. you may see natural improvements in your hormone levels, strength, size, and overall health.. if not.. you can try maintaining this and looking into a restart protocol. HCG and Clomid dosed and timed properly to act cohesively in an effort to induce recovery.
If this fails..
THEN
Look into replacement therapy.
We're talking up to 6 mos. of dedication to recover naturally. And if that fails, 1.5-2 mos. trying to induce recovery with medications..
21 marks the average spot a male is finished developing physically. 25 marks it mentally. TRT and any other protocol of AAS use will cause shutdown of certain sectors of the brain.. this is frowned upon on a developing mind. I hope you can see the depth of that for your future's sake. Cut losses after you've exhausted all efforts.
I did a cycle at 23 going on 24. October cycle start, birthday was in November. I started TRT in June - July at 24 y.o. I was low before starting, and low after 2-3 months beyond that PCT. I was still eating 6-7k calories of whole foods ED, training ED, sleeping 8 hrs.. working my normal work.. So I made the dive then. Don't regret it a bit, and I wouldn't think less of you for feeling the same way.. but you're a few important years younger than I was then. Just be careful.
Good luck in any endeavor, hope I've helped.