Symptoms follow a periodic trend closely tied to the menstrual cycle. Symptoms tend to peak in the days to weeks before each period and decrease afterwards. At peak, breasts may feel full, heavy, swollen, and tender to the touch. No complications related to breastfeeding have been found. Pathophysiology
The exact mechanism of the condition is not fully understood, though it is known to be tied to hormone level fluctuation, the condition usually subsides after menopause and is closely related to the menstrual cycle. Post-menopausal under hormone replacement therapy have also reported symptoms of fibrocystic breast changes indicating hormones may play a major role. This condition is an accumulative process, partly caused by the normal hormonal variation during a womans monthly cycle. The most important of these hormones include: estrogen, progesterone and prolactin. These hormones directly affect the breast tissue by causing cells to grow and multiply. Other hormones such as TSH, insulin, growth hormone and growth factors such as TGF-beta exert both direct and indirect effects by amplifying or regulating cell growth. Chronic hormonal fluctuations eventually produce small cysts and/or areas of dense or fibrotic tissue over the years. By the age of 30, multiple small cysts and breast pain may arise. Larger cysts usually do not occur until after the age of 35. Over time, presumably driven by aberrant growth signals, such lesions may accumulate epigenetic, genetic and karyotypic changes such as modified expression of hormone receptors and loss of heterozygosity.
According to one study, based on modeled concentrations, pre-Anthropocene tissue-bound levels in fish may not have differed markedly from current levels. However, based on a comprehensive set of global measurements, the ocean contains about 60,000 to 80,000 tons of mercury from pollution, and mercury levels in the upper ocean have tripled since the beginning of the industrial revolution. Higher mercury levels in shallower ocean waters could increase the amount of the toxicant accumulating in food fish, exposing people to a greater risk of mercury poisoning. See also
Canadian Reference Materials include some with methylmercury, e.g. DORM
Dimethylmercury, mercury with a second methyl group
Ethylmercury, a related cation
Mercury poisoning
Mercury regulation in the United States

References
External links
ATSDR - ToxFAQs: Mercury
ATSDR - Public Health Statement: Mercury
ATSDR - ALERT! Patterns of Metallic Mercury Exposure, 6/26/97
ATSDR - MMG: Mercury
ATSDR - Toxicological Profile: Mercury
National Pollutant Inventory - Mercury and compounds Fact Sheet
Methylmercury-in-fish exposure calculator provided by GotMercury.Org, which uses FDA mercury data with the EPAs calculated safe exposure levels. Methylmercury Contamination in Fish and Shellfish Archived 2013-11-02 at the Wayback Machine
nytimes.com, Tuna Fish Stories: The Candidates Spin the Sushi
U.S. Environmental Protection Agencys mercury site
U.S. Geological Surveys mercury site Archived 2013-11-02 at the Wayback Machine
Environment Canadas mercury site
Health Canadas mercury site
International Conference on Mercury as a Global Pollutant 2006-Madison, WI USA  2009-Guizhou, China  2011-Halifax, NS Canada