There are a number of causes of hyperhidrosis that I will summarize blow. One alternative way of describing the causes of hyperhidrosis are either thermoregulatory sweating or emotional sweating. The former regulates body temperature and is primarily controlled by the hypothalumus. The latter is primarily controlled by the limbic system.
According to various studies, for anywhere from 20 to 50 percent of patients hyperhidrosis represents a genetic condition that is not linked to any underlying medical condition. Nor is it due to any kind of side effect from medications. This type of sweating presents itself it specific focal areas of the body (e.g., sweaty palms, sweaty feet, craniofacial hyperhidrosis). It seems like Asian and Jewish people have a much greater rate of prevalence of hyperhidrosis than other races. No wonder that the world's most experienced ETS surgeon, Chien-Chi Lin, is based in Taiwan, and is responsible for the major innovations in ETS surgical technique since the 1990s. Numerous other Asian surgeons have published large sample size studies on hyperhidrosis, and the information on the web from China is not even remotely comprehensive due to translation and information release issues. Also, Israel, which is about 75 percent Jewish (a 50/50 split between Ashkenazi and Sephardic/Mizrahi per wikipedia), accounts for a very high number of ETS surgeries every year relative to its small population. Several of the most experienced surgeons in the US also initially learned to perform sympathectomy in Israel. Finally, based on the e-mails I have received over the years and based on personal youtube videos on this condition, it seems like Filipinos have a significantly higher rate of hyperhidrosis sufferers than most other ethnicities. I think that their genetic composition is very different from other Asian peoples' genetic composition.
For most people who have primary focal hyperhidrosis, the condition starts in puberty. Hyperhidrosis in children who have not reached puberty should not be treated with surgery, since the excessive sweating can sometimes diminish with time and age.
For some, the sweating is caused by secondary hyperhidrosis, which means that there is an underlying medical condition causing perspiration. This phenomenon is called diaphoresis. Often, secondary hyperhidrosis causes excessive sweating all over the body (generalized) rather than in localized regions such as the palms/feet/face/armpits. The most cited medical condition that can cause hyperhidrosis is hyperthyroidism. Other possibilities include adrenal gland tumors or dysfunctions, hyperpituitarism, and spinal cord injuries. Temporary hyperhidrosis can occur from all kinds of medical and life conditions ranging from angina to menopause, as well as from side effects from taking medications such as propranolol, physostigmine, pilocarpine, antidepressants, and selective serotonin reuptake inhibitors.
For the remaining hyperhidrosis sufferers, there is no known genetic or medical condition related explanation. I fall under this category it seems. Posters on various internet discussions have postulated a number of theories to explain the development of hyperhidrosis and/or facial blushing in this group, ranging from a hyperactive personality in childhood, to excessive frequent stress or fear when young, to excessive pressure on the back near the sympathetic nerves via wearing heavy backpacks in childhood, to playing too many competitive sports in childhood, to swimming a lot in chlorinated water, to aluminum and lead related nervous system damage, and many more reasonable as well as totally wacky reasons. There is no conclusive evidence that any of these factors can lead to a permanent change in the functioning of your sympathetic nervous system.
Localized hyperhidrosis (as most visiting this site probably suffer from) is not due to nervousness, excess heat and humidity, or obesity as would be expected. Those conditions can sometimes make the problem worse, but do not cause localized hyperhidrosis. When my hands used to sweat profusely before I had ETS surgery in 1998, they would do so even if I was by myself in an air conditioned room with no reason whatsoever to be nervous. Moreover, in 1998, I was 5 feet 11 inches tall and weighed 145 lbs. Quite underweight for a male. Localized hyperhidrosis is a very strange problem that does not seem to have an evolutionary benefit or explanation. Why do humans even have the majority of their sweat glands in the hands and feet? Why do some people never ever sweat from their hands or feet despite having so many sweat glands there?