Gross thiamine deficiency presents a complex of systemic symptoms known as beriberi, resulting in serious health consequences. A common affliction of those on military rations or restricted diets 100 years ago, today gross thiamine deficiency is generally preceded by specific, prolonged, and unhealthy lifestyle habits.
Alcohol consumption decreases thiamine absorption in the small intestine (leading to the prevalence of beriberi in chronic alcoholics).
The tannins in tea and coffee can also lead to poor absorption and regular consumption of tannin-containing drinks can lead to a mild deficiency of thiamine.
Water treated with chlorine(like some bottled and public waters) can cleave thiamine, destroying the vitamin.
Thiamine is decreased by sulfites, naturally occurring in food or added to food and food packaging - ingredients labeled with sodium dioxide or “sulfite” as part of the name.
Oral contraceptives (birth control) are suspected to lower thiamine levels.
In a large study of women, when given 100mg of thiamine daily, dysmenorrhea (painful menstruation) was alleviated in 87 percent. 
Furosemide (a “water pill”) is cited in inducing increased urinary excretion of thiamine. Frequently given to elderly patients - a population with naturally decreased absorption through digestion - this drug compounds the likelihood of suboptimal thiamine levels. In a study of elderly patients diagnosed with congestive heart failure and prescribe furosemide - daily thiamine was supplemented resulting in improved left ventricular ejection fractions. In other words, supplemental thiamine improved their heart’s ability to circulate blood despite the chronic heart disease. Thiamine supplementation is shown to decrease high blood pressure in those populations at risk for thiamine deficiency, the old and chronically ill. 
Postoperative patients are often found to be thiamine deficient due to dietary restriction, prescribed supplemental food sources, and larger nutritional demand from the body after surgery - particularly the demand for energy and cellular regeneration. 
In combination with other B vitamins, vitamin B-1 may decrease sciatica pain. Studies in animals show it decreases nerve inflammation and pain.  Thiamine can also be beneficial in alleviating peripheral neuropathy and neuritis (nerve pain and inflammation). 
Suboptimal levels of thiamine cause a decrease in the activity of the enzymes sucrase, maltase, and lactase in rats. Deficiencies of these enzymes play a role in the pathology of Crohn's disease. 
Suboptimal thiamine may cause cognitive impairment in patients with epilepsy. After supplementation in these populations, testable IQ levels were seen to increase.
In a test survey of elderly patients with a primary presentation of dementia, over half had plasma levels indicating a deficiency in thiamine. Thiamine levels were significantly lower in Alzheimer's patients than dementia patients. Those with Alzheimer's disease also exhibited a decreased activity of thiamine dependent enzymes in the brain. While thiamine is known to be important for brain health it is still inconclusive if supplementation of thiamine after diagnosis will decrease the symptoms of either disease.
A study of college-age females with normal thiamine status reported a significant increase in mood after two weeks of supplementation with 50 mg of thiamine daily. This suggests that even with adequate levels of thiamine, additional thiamine supplementation may improve symptoms of depression.
When selecting a thiamine supplement, there is some evidence that the fat-soluble forms of the vitamin are better absorbed than the water-soluble forms. However, finding a reputable and quality source of thiamine is most important in therapeutic effectiveness.