Canned tuna is a great source of protein but has two big problems normally: a) it is stored in BPA-lined cans and b) it is a high mercury (methylmercury) fish. I have had a couple of men on the Peak Testosterone Forum who did not feel well after consuming tuna for a significant period of time. You can read about a documented case of mercury poisoning from canned tuna in a well-known actress (Denise Zuniga) here: Testosterone and Mercury. Mercury causes many nasty issues, including binding to many enzyme systems and hard-to-remove storage within neurons.
So how do you find out if you have mercury overload? Well, there essentially two ways to do it, depending on the stage of your mercury overload from what I understand:
a) an initial overload stage where high levels of mercury can show up in your plasma and hair or
b) a storage stage where mercury is bound and stored in various tissues and disrupting multiple metabolic pathways (including potentially testosterone production)
I deal with the longer term stage here: Hair Testing for Mercury (Stage II). On this page I want to deal a) where the body is loading up on ,mercury and it is readily detectable in your blood and hair.
Unfortunately, one of the senior members on Peak Testosterone Forum discovered this was an issue with his mother. The forum member (EH) actually found out about the high mercury levels by testing his mother's hair. He wasn't actually looking for mercury but found out about by testing. Here is what he wrote:
"My mother had high aluminum and mercury levels last year when she tested her hair (done via ICP-MS method by DirectLabs). After a long discussion about her diet, she mentioned that she was eating Kirkland [Costco] Signature Canned Tuna frequently. I asked her to discontinue eating any canned tuna for a while and test again. She just tested again after 9 months of not eating the tuna. Her new results were dramatically improved. Her aluminum numbers dropped by 50% and her mercury numbers dropped by 90%. It is well known that tuna are contaminated but I had never seen personal evidence of it until now." 
In an email, EH elaborated that his mom stated that she had been consuming canned tuna at least 4-5 times per week. This is way too much and, as you can see, there should be warning label on tuna! It simply should not be eaten more than once per week at the most according to most of the experts I have read. Furthermore, a certain percentage of the population has a glutathione-related mutation that makes them very vulnerable to heavy metals. Their mercury levels will spike tremendously after consumption of a high mercury food like this.
The clear point in all of this is that hair testing can literally save you years of poor health and possibly even chronic disease.
EH very graciously shared his Mom's charts before and after so that you can see the tremendous improvement:
NOTE: Hair testing was done via Direct Labs / Doctor's Data. (I have no affiliation.)
CONCLUSION: Hair testing can be a powerful way to find potential issues and toxicities. Hair testing likely saved this forum member's mother years of poor health. For more information, see these links: Summary Pages on Hair Testing and Inexpensive Self-Testing Labs.