Testosterone and Heat
It is no secret that heat near the testes (testicles) can lead to greatly
reduced fertility. Everything from spas to laptop can lead to sperm cell
death among other things. But what about testosterone levels themselves?
Can heat near the testicles lead to decreased testosterone levels?
The answer to that is a little more complex and we have to go to animal studies
to get some answers and insight. First of all, heat will definitely shrink the
testes, something most men are not too excited about. In a study of rams, heat
actually shrunk the testes by about a third by weight. 
This is particularly remarkable, because the rams were not subject to ultrahigh
heat, but rather constant heat in the 28-23 degrees Celcius range (82-90 degrees
Fahrenheit) continuously for two weeks.
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Of course, shrunken testicles is a very bad sign. But does this mean
testosterone was actually lowered? A study on increasing heat using rats
that FH and LSH rose. Now testosterone, interestingly enough, did not change,
but the testes became less responsive. So it is as if there was a "partial
secondary hypogonadism" even though testosterone levels did immediately fall.
 In fact, the authors concluded that "it appears that the testosterone
produced by Leydig cells from heated testes may not be secreted as effectively
as in normal testes."
So, obviously, something is going on. And, in fact, one study actually
looked at using a combination of exogenous (applied from outside the body)
testosterone plus heat to create a male contraceptive.  It was so effective
that it almost reduced sperm counts to zero.
But does heat affect testosterone levels.
One study on boars shows that initially heat may not affect testosterone levels
 However, eventually baseline T levels are actually decreased. In
other words, it appears there may be a time lag from the application of the heat
to the time that testosterone actually decreases. This was verified in the study
on rams that showed that testosterone decreased significantly after a two week
period.  One study on monkeys verified the same thing: a time lag loss
of testosterone after 6 days of 30 minutes heat treatment of the testicle area.
J OF ANIMAL SCIENCE , 1971, 33(4), "EFFECT OF ELEVATED AMB IENT TEMPERATURE ON
TESTIS AND BLOOD LEVELS AND IN VITRO BIO - SYNTHESIS OF TESTOSTERONE IN THE RAM"
2) Int J Androl, 1988 Feb, 11(1):61-72, "Effects of local heating of the testes
on the concentration of testosterone in jugular and testicular venous blood of
rats and on testosterone production in vitro"
3) Endocrinology, Apr 1, 2000, 141(4):1414-1424, "Testicular Heat Exposure
Enhances the Suppression of Spermatogenesis by Testosterone in Rats: The
“Two-Hit” Approach to Male Contraceptive Development"
4) Animal Reproduction Science, Aug 1984, 7(5):391–403, "Effects of acute and
chronic testicular hyperthermia on levels of testosterone and corticosteroids in
plasma of boars"
5) Journal of Andrology, November/December 2002, 23(6), "Mild Testicular
Hyperthermia Induces Profound Transitional Spermatogenic Suppression Through
Increased Germ Cell Apoptosis in Adult Cynomolgus Monkeys (Macaca fascicularis)"