Around 40% of serum total testosterone is bound to sex hormone binding globulin (SHBG). Testosterone bound to SHBG does not diffuse into cells.
If the amount of free testosterone is less than expected based on the total testosterone, then the serum SHBG may be increased.
If the amount of free testosterone is more than expected based on the total testosterone, then the serum SHBG may be decreased.
Conditions associated with increased serum SHBG:
(1) older age
(2) hyperthyroidism
(3) HIV
(4) malnutrition or caloric restriction
(5) anorexia nervosa
(6) anticonvulsant therapy
(7) estrogen exposure
(8) genetic variation (Asp327>Asn)
Conditions associated with decreased serum SHBG:
(1) obesity
(2) hypothyroidism
(3) type 2 diabetes
(4) nephrotic syndrome
(5) acromegaly
(6) steroid therapy
(7) progestin exposure
