Your body produces a complex array of hormones. Some, like thyroid hormones, are best tested in the blood. Others, like steroid hormones (cortisol, DHEA, estrogen, progesterone, and testosterone) can be accurately tested in either the blood or the saliva. The most clinically significant type of steroid hormone to measure is the unbound (unattached to a protein carrier), as opposed the bound (attached to a protein carrier).
The reason for this is that a bound hormone is actually inactive until it gets released from its protein carrier, at which point it becomes an active unbound. In the blood about 98% of the steroid hormones are bound and only about 2% unbound, but in the saliva it’s the opposite. Nearly all of the steroid hormone in saliva is unbound.
While it is possible to measure the unbound steroid hormone in the blood, it is a time-consuming and complex lab procedure that is much more expensive than salivary testing of unbound steroid hormones. This makes saliva testing a very affordable way to study active steroid hormone levels.