Methyl mercury, a byproduct of the industrial age, has permeated our environment. The planet is covered in coal-based power plants and these spew mercury into the atmosphere, which then settles over the entire planet. Every mountain, valley, ocean and stream becomes polluted leading to a contaminated food supply for us.
Methylmercury accumulates in the testosterone-producing Leydig cells of the testes , the brain and pituitary. Several studies have shown workers occupationally exposed to mercury have lower testosterone levels. 
Keep in mind that it doesn't take much to lower testosterone. Mercury is both toxic and powerful. Denise Zuniga, the actress who accidently mercury poisoned herself from eating too many cans of tuna per day, only had tissue levels about twice that of a normal person.  She has devoted an entire page on her site to the subject for those interested. 
Mercury can affect many of our body's systems, including fertility  and thyroid  function as well. In other words, all of your hormones can be affected by this deadly toxin. And it's important to mention that Mercury's deadly affect on your neurons is a known IQ-lowerer. Recent news attention has been given to mercury's ability to lower the IQ of children in the womb. Experts estimate that about 8% of pregnant women have high enough levels to damage (slightly) the brain of their fetus. 
So how can you best preserve your testeosterone and other hormones from the ravages of this heavy metal?
NEWS FLASH: One of our Peak Testosterone Forum members documented a case study, using hair testing, where his Mom clearly was experiencing mercury overload from consuming 4-5 cans of tuna every week. See this page for the details: Hair Testing and Mercury.
1. Fish. Do not consume fish more than once or twice per week. Most experts recommend fish only once or twice per week because of the methylmercury content. I cover this in greater detail in my link on Fish Safety. One story on this page is about a Discover article writer who found that his mercury levels went very high after consuming fish. The reason, he found out, was that he had a negative, but quite common genetic mutation that governs his glutathione levels.
2. Corn syrup. Avoid corn syrup. Researchers just announced that mercury contamination is extremely common in high fructose corn syrup: 50% of products tested by researchers were contaminated!  The issue apparently is that there are no standards for mercury residue in hydrochloric acid and hydrochloric acid is used to make high fructose corn syrup. NOTE: You should be safe consuming Fish Oil as a recent Consumer Lab Study looked at over 50 fish oil brands and found all were mercury-free.  (Fyi: PCBs and lead were also in check.)
3. Glutathione. Boost your body's glutathione levels. Glutathione is your natural detoxification chemical. I have dedicated a whole page, How to Naturally Boost Your Glutathione Levels, to the subject.
4. Coriander, Parsley and Cilantro. These three herbs and spices are known for binding easily to mercury and some experts feel these can aid in detoxing the body from mercury, lead and other heavy metals. 
5. Watch Those Mercury Fillings. Removing mercury fillings dramatically reduces plasma mercury levels according to a couple of studies.  Should you remove them. Talk to your doctor and dentist about that one. However, mercury loading looks like a potential issue, especially for those who chew gum or grind their teeth.  One animal study showed that 12 fillings could actually impair kidney function short term.  What if you have 2 or 3 fillings leaching out mercury over decades? That is a difficult question, but it may have to do with your genetics: see #1.
6. Undenatured Whey. The standard wheys in most stores are loaded with nasty excitotoxins and, just as important, do not boost your glutathione levels as real whey does. I cover this in more detail in my link on Undenatured Whey, which is a powerful glutathione booster.
1) Food Chem Toxicology, 2008, 46:270-279
2) J Environmental Health, Jan 2009, 8:2"Mercury from chlor-alkali plants: measured concentrations in food product sugar", Renee Dufault, et al; Institute for Agriculture and Trade Policy, Jan 26 2009, "Much High Fructose Corn Syrup Contaminated with Mercury New Study Finds: Brand Name Food Products Also Discovered to Contain Mercury".
4) Reproductive Toxicology, 1991, 5(3):205-209, "Ultrastructural demonstration of mercury in sertoli and leydig cells of the rat following methyl mercuric chloride or mercuric chloride treatment"
5) Reproductive Toxicology, Sep-Oct 1993, 7(5):469-475, "Autometallographic detection of mercury in testicular tissue of an infertile man exposed to mercury vapor"
6) Occup Environ Med, Aug 1994, 51(8):536 540, "Endocrine function in mercury exposed chloralkali workers"
7) Comptes Rendus de l'Academie Bulgare des Sciencesi, 53(12):91, "Mercury Vapour and Some Reproductive Parameters in Human"
12) Toxicol Appl Pharmacol, 1997 May, 144(1):156-62, "Mercury in saliva and feces after removal of amalgam fillings"
13) J Dent Res, 1996 Jan, 75(1):594-8, "Long-term Use of Nicotine Chewing Gum and Mercury Exposure from Dental Amalgam Filling"
14) AJP - Regu Physiol, Oct 1 1991, 261(4):R1010-R1014, "Mercury from dental 'silver' tooth fillings impairs sheep kidney function"