Studies show that hair follicles are very sensitive to thyroid hormone levels. This is because the follicular cells of hair have receptors for thyroid hormone, particularly for triiodothyronine (T3). In studies when these receptors are exposed to excess T3 hair growth is stimulated.

More than 30 percent of people with hypothyroidism report diffuse hair loss, which is a variety of telogen effluvium. With replacement hormone, hair growth is promoted but not always to its previous extent. Anecdotal, but not scientific, reports suggest that glandular replacement hormone such as Armour is better at encouraging hair growth than synthetic forms of levothyroine.

Smaller numbers of people with hyperthyroidism can experience hair loss. This suggests that having less than optimal levels of thyroid hormone upsets the normal growth cycle of hair. Outside of this optimal range, hair follicles can shut down and enter an extended telogen resting state. Here, normal hair loss occurs but new hair growth is halted.

Alternately, occasionally people with hypothyroidism or hyperthyroidism report experiencing hypertrichosis - an excess of hair growth. This might be expected in people with mild hyperthyroidism, where a modest increase in thyroid hormones may stimulate the hair follicles into increased growth but not be so high as to be toxic for the cells. But why some people with a lack of thyroid hormone production should also experience hypertrichosis is not clear.

Both hypothyroidism and hyperthyroidism can also cause malabsorption, which leads to nutrient deficiencies that cause hair loss. Deficiencies of omega-3 oils, biotin, vitamin E (mixed tocopherols) and vitamin B all affect hair growth and its texture. April 25, 2011