Similarly, vaccinations, crash dieting, physical trauma such as being in a car crash, and having surgery can sometimes be a shock to the system and a proportion of scalp hair follicles go into hibernation. As the environmental insult passes and the body recovers, the TE subsides and there is new hair growth.
Some drugs may also induce TE, especially antidepressants. Often a switch to a different drug resolves the issue.
More persistent insults can result in more persistent TE. For example, a chronic illness may lead to TE. Arguably, the two most common problems are chronic stress and diet deficiency. Many dermatologists believe chronic stress can gradually exert a negative effect on hair growth and lead to persistent TE. Research with animal models has provided evidence to back up this claim. There does indeed seem to be a link between stress, a change in hair follicle biochemistry, and more hair follicles entering a telogen resting state.
Whether dietary problems are causing TE in North America is hotly argued among dermatologists. A lack of a mineral, vitamin, or essential amino acid can certainly cause TE, such as with people in third world countries where diets can be completely deficient in one or more nutrients. Animal experiments also provide supporting evidence.
In first world countries the average diet is rarely completely deficient in a particular vitamin or mineral. However, some dermatologists claim that with a reduction in red meat intake and a preference for vegetarian diets, some individuals are not getting a balanced intake of all the nutrients required for good hair and overall body growth. In particular, there are claims that women may be deficient in their iron intake. Why women specifically? Because women lose iron at regular intervals as a result of menstruation.
Some dermatologists believe that as we now eat less red meat, a key source of iron, some people are not eating enough iron and TE is the result. Other potential deficiencies of the modern North American diet -- such as a lack of zinc, amino acid L-lysine, or vitamins B6 and B12 -- have also been suggested to contribute to TE.