Thanks to dwkdnvr for also noting that iodine is also very important for hormonal production (including testosterone). I personally drink a lot of milk (I've not participated in any dairy restrictions as part of my diet), use TONS of salt and other spices on my food, and get a good deal of seafood, so I can't imagine I have low iodine levels.
Regarding tests for T-levels, people should bare in mind that there is substantial testosterone fluctuation throughout the week, month, and even the day. Levels are almost always higher in the morning or after a period of sexual arousal, for instance. So, just as most of us have learned to be skeptical when a doctor says, "High cholesterol, so buy statins!" also be leary if he's saying you have low testosterone and he's hawking synthetic testosterone to you. (It's also worth noting that there's a healthy variance between all people, and we're not all like Kobe Bryant or whatever.)
It's actually horrifying what's happened with testosterone and then replacement therapy: the powers-that-be have insidiously robbed Western males of it....and now they're trying to sell it back to us. This is like somebody stealing your car and then offering to give it back to you if you pay him money.
I don't have any test results but I'm pretty sure mine has gone up significantly. Hair has started coming back to my arms and legs. (I'm 69.) I just "feel" like I have more. And then there is that built in gauge that men have in the morning.
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