400 is not too low, esp. for middle age and past. If you feel good and rock hard there is no need to mess with T. Everybody has individual levels and it's not really related to your libido. You can be a total fucking stallion with 300+, and in that case you don't want to double it as it may make you uncontrollable, let alone health risks. Use it judiciously
Some further elaboration for the readers who are probably annoyed already.
I'm not an "Old fuck"; they kicked me out of the La Paz special Walmart checkout line: "This is for people over 50." I laughed and told the cashier I owed her a tip.
More disclosure: Viagra wasn't working that well so I went for a total package of treatment; 3 years at $5000/year. You've heard of the Six Million Dollar Man; I have the Fifteen Thousand Dollar Dick.
My libido wasn't bad before, but my higher T is definitely affecting my dirty mind.
An other interesting thing I'm taking is Oxytocin, I don't know the dose or units.
I had other treatments which were fascinating.
If Viagra and Cialis work and you're not sitting on the couch, you're good.
My take on my friends and brother is I can't imagine they even fuck their partners so I don't talk too much about this with them.
An interesting aspect of all of my checkups and blood tests over age 65 is NO doctor measured my Testosterone level.
Last July I went to a "Men's clinic" type place in Florida and my Testosterone was measured: 404 (I don't recall the units).
I started taking Enclomiphene citrate 12.5 mg for a month or so and my Testosterone went to 655.
Enclomiphene gets your own testosterone production up.
If you get on actual Testosterone, your body stops making it, so you have to take it forever. So, Enclomiphene is preferred.
Another tip: taking 5mg daily of Tadalafil (Cialis) improves general circulation, not just down below.
Factoid: higher Testosterone also helps ED meds work better, e.g. Sildenafil (Viagra) and Tadalafil (Cialis).
Tip #2: If you're younger don't take ED meds or you'll think you need them and they'll become a bit of a crutch.