SHBGSex hormone-binding globulin
The carrier protein that decides how much of your testosterone is usable.
Sex hormone binding globulin is a protein made by the liver that binds tightly to testosterone and, less tightly, to oestradiol. Bound hormone is not available to tissues, so SHBG effectively controls how much of your sex hormone is actually active.
That makes it essential context for interpreting a testosterone result. A total testosterone measurement counts bound and free together, so a man with high SHBG can have a reassuring total and a low free level, and a man with low SHBG the reverse.
Its level moves with several conditions. It falls with obesity, insulin resistance, type 2 diabetes and hypothyroidism, and rises with ageing, hyperthyroidism, liver disease and oestrogen exposure including some oral contraceptives.
Low SHBG is therefore something of a metabolic marker in its own right, associated with insulin resistance and the risk of developing type 2 diabetes, independent of testosterone.
Clinical guidance accounts for this. When total testosterone is near the lower limit of normal, or when a condition affecting SHBG is present, free testosterone should be measured by equilibrium dialysis or calculated with an accurate formula rather than inferred.
That is why direct to consumer testosterone panels that report a total figure alone are the weakest form of the test, and why an unexpected result deserves proper interpretation rather than a supplement.
In practice, if testosterone is being investigated, SHBG belongs on the same request form, and the whole picture belongs with a doctor.