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Testosterone in its own right does not have much to do with facial or body hair. It is DHT and/or other 5-alpha reduced metabolites of testosterone that are primarily responsible for hair growth.

There is a condition called 5-alpha reductase type 2 deficiency (i.e. deficiency of the type 2 5-alpha reductase enzyme that converts testosterone into DHT). Males with this condition have ambiguous genitalia at birth and are often raised as girls. At puberty, however, they often produce supra-normal amounts of testosterone, change their gender identity, and develop deep voices and large muscles, but grow little in the way of facial or body hair [1].

Genetic differences in body/facial hair are also important, because persons with 5-alpha reductase deficiency in Papua New Guinea tend to have more hair than those in the Dominican Republic [2].

Studies on persons without the above deficiency have found a rough correlation between voice depth (a testosterone characteristic) and “leadership” but not between hairyness and “leadership” [3]; other studies have found a rough correlation between hairyness (a DHT characteristic) and IQ (and homosexuality) [4]. Political leaders tend to have deeper voices whereas scientists tend to have more body hair.

Another important thing to note is that pubertal levels of DHT are more important for hairyness than current levels. Treatment of some 5-alpha reductase deficient subjects with DHT after puberty did not reliably increase facial or body hair in all of the subjects, and had little effect on the size of the penis, whereas treatment before puberty caused the penis to double in size [5]. Treatment of adult male-to-female transsexuals with estrogens, which caused androgen levels to plummet immediately, did not cause any reduction of facial hair growth for many months (and it continued at a slower rate even after that), and treatment of adult female-to-male transsexuals with androgens over an entire year did not result in a degree of facial hair growth comparable to normal male levels [6].

One factor that has been found to cause a high-testosterone, low-DHT phenotype in males in the absence of 5-alpha reductase deficiency is coeliac (celiac) disease. This hormone profile stems from an androgen resistance that reverses once a gluten-free diet is followed. The facial hair of male coeliacs was examined, and it was found that they had reduced linear facial hair growth (which correlated with lower DHT levels) but an increased density of hair follicles (which correlated with higher testosterone levels) [7].

So, the conclusions from the above are:

– Your doctor is wrong to say that beard growth is a sure sign that your testosterone level is not too low

– Genetic factors can influence the amount of body and facial hair independent of androgen levels

– Testosterone appears to increase the density of hair follicles but has little or nothing to do with body and facial hair growth

– DHT (probably along with other 5-alpha reduced androgens) promotes body and facial hair growth

– Pubertal DHT is more important than current DHT for hair growth

– High levels of DHT and other androgens do not promote a normal degree of hair growth in adulthood if they were not high in puberty

– Hair from already present follicles continues to grow even in the almost total absence of DHT and other androgens (thus, your beard doesn’t even guarantee that you have normal or high DHT levels)

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Source(s):

[1]:http://www.ncbi.nlm.nih.gov/pubmed/12573?

[2]:http://www.ncbi.nlm.nih.gov/pubmed/18317?

[3]:http://www.ncbi.nlm.nih.gov/pubmed/10859?

[4]:http://www.ncbi.nlm.nih.gov/pubmed/15677?

[5]:http://www.ncbi.nlm.nih.gov/pubmed/86066?

[6]:http://jcem.endojournals.org/content/85/?

[7]: http://www.ncbi.nlm.nih.gov/pubmed/71264?