It would be comforting to think that a simple blood test could identify androgen deficiency. Unfortunately, this is not the case.
Widespread disagreement exists over what the normal range of testosterone levels are and what, exactly, should be measured in the blood to assess androgen deficiency.
The existing 'normal' range for total testosterone is based upon statistical analysis of pooled samples from all men, including those who might have PADAM. So 'normal' testosterone levels are not necessarily the same as healthy levels.
Testosterone is released into the bloodstream in pulses, and levels vary through the day (diurnal variation). In general, the testicles release more testosterone in the morning than later in the day.
Blood samples should therefore be taken between 8am and 10am, and at least two separate, consistent results are needed to establish that there is a problem with testosterone levels.
About 60 – 70% of the total testosterone is tightly bound to a protein, present in the blood, called sex hormone binding globulin (SHBG). This protein-binding is a common way in which hormones are transported in the bloodstream and it is effectively a circulating store of testosterone. The testosterone only becomes active when the link to SHBG is broken, and this is a process which occurs at a certain rate all the time.
Older men produce relatively more SHBG, as do heavy drinkers and men with thyroid disorders, thus reducing the amount of 'free' testosterone.
Another 30 – 40% of the total testosterone is more loosely bound to another protein, called albumen. Testosterone bound to albumen is also inactive, so free testosterone probably accounts for only 1-2% of the total.
Measurement of total testosterone is therefore a poor measure of active testosterone. Free testosterone levels are expensive to measure and are not widely available.
Free Androgen Index (FAI = total testosterone/SHBG x100) is an alternative measure of androgen state that is not as reliable as free testosterone, but is better than relying solely on total testosterone.
All this is confusing for doctors, too!