The big picture: SHBG (sex hormone-binding globulin) is a protein produced by the liver that binds to sex hormones so your tissues can’t use those hormones. It is especially useful in evaluating men who have symptoms of low testosterone and women for excess testosterone production.

SHBG helps control the amount of sex hormones that are actively working in your body.

SHGB transports the hormones testosterone (an androgen), dihydrotestosterone (DHT) (an androgen), and estradiol (an estrogen) in the blood as biologically inactive forms.
Changes in SHBG levels can affect the amount of sex hormones used by the body’s tissues.
Measuring SHBG levels can provide information about how much of your testosterone is active in the tissues of your body.

Abnormally high SHBG levels means less of your total testosterone is “free” and thus available for your tissues to use. So your tissues may not be getting enough testosterone.

Abnormally low SHBG levels means more of the total testosterone is likely to be available for your body to use.

There is increasing evidence that low SHBG levels are an independent correlate of the metabolic syndrome / insulin resistance and also an independent risk factor for diabetes.
SHBG is lower at all ages in obese men than in nonobese men.

For women, hormonal birth control contraceptives are known to increase the production of SHBG, and can result in abnormally high SHBG levels. Different forms of contraceptives have different effects on SHBG levels.

IUDs have the least impact, followed by oral contraceptives, and then ring/patch contraceptives. This is because IUDs contain lower amounts of hormones, while ring/patch contraceptives deliver higher hormone doses.
The choice of hormones used in contraceptives also affects the level of increase in SHBG, e.g. progestin- or progesterone-only medications are typically associated with the lowest increase in SHBG.