Brief Answer:
Testosterone

Detailed Answer:
Free testosterone as a blood test is reliable only when done by a method called 'Equilibrium dialysis'. Few laboratories use this method. So check with yours to see of this technique was deployed.

If not, there is an indirect way to estimate free testosterone in the blood. This requires the following three blood tests:

Total testosterone
Albumin
SHBG (Sex Hormone Binding Globulin)

Then there are free online calculators that allow you make this conversion after plugging in these values.

The normal range for free testosterone can vary a bit from lab to lab but a lower limit cut off 5.7 to 6.5 is generally acceptable.

Furthermore, the timing of the blood sample for total and free testosterone measurements is important. It is best drawn before 9 am. Fasting is not necessary.

Interpreting testosterone lab reports is not as straightforward as reading the numbers. A host of clinical symptoms and signs are required to put the numerical value in perspective. This entails a systematic assessment in person by an endocrinologist, including a detailed history and thorough physical examination, followed by further tests such as
CBC
CMP
PSA
TSH
Free T4
FSH
LH
Ferritin
Prolactin

These are just examples of tests that may be considered by the treating physician based upon findings of other basic tests.

Free testosterone is the actually active testosterone in the body. However, for many years total testosterone has been studied and hence there is more experience and familiarity in reporting total testosterone in general.

But free testosterone is increasingly becoming standard of care for testing and diagnosing 'Androgen deficiency in Males'

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Brief Answer:
Follow up

Detailed Answer:
Yes, Endocrinologists are best equipped to understand and manage this condition. You may also want to think of what gender of the doctor you would be more comfortable discussing these issues with

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