It is suggested that bioavailable testosterone represents the fraction of circulating testosterone that readily enters cells and better reflects the bioactivity of testosterone than does the simple measurement of serum total testosterone. Also, varying levels of SHBG can result in inaccurate measurements of bioavailable testosterone. Decreased SHBG levels can be seen in obesity, hypothyroidism , androgen use, and nephritic syndrome (a form of kidney disease ). Increased levels are seen in cirrhosis , hyperthyroidism , and estrogen use. In these situations, measurement of free testosterone may be more useful.
I can confirm this to be true. I get my testosterone checked regularly along with my TSH numbers. For some reason my TSH is never a consistent number and my testosterone levels have been in the crapper for a long time. Even with testosterone supplements my levels didn’t improve much, if at all. Last July my doctor said my PSA numbers were a bit high so he told me to stop taking the Andriol, the testosterone supplement. Fast forward to December when I started taking 2000 IU of vitamin D per day. I had my blood tested again a few weeks ago at the end of February and my doctor said my testosterone levels were higher than they’ve been over the last few years while taking supplements.
But I'm not more aggressive—a behavior change often tied to testosterone. That's not surprising to Robert Sapolsky, ., a neuroendocrinologist at Stanford University and a leading researcher on stress and behavior. "It's really not the case that testosterone 'causes' aggressive behavior," he says. "Instead, it makes the brain more sensitive to social cues that trigger aggression. And in support of that, a guy's testosterone level isn't a very good predictor of how likely he is to be aggressive."