Sex Hormone Binding Globulin

Sex hormone binding globulin, known as SHBG, is a protein produced mainly by your liver that acts as a transport vehicle for sex hormones, particularly testosterone and oestrogen, as they travel through your bloodstream. While SHBG is not a hormone itself, it plays a crucial role in determining how much of your sex hormones are actually available for your body to use. Understanding your SHBG level adds important context to your testosterone and oestrogen results and can reveal subtleties that those markers alone might not show. Imagine SHBG as a fleet of taxis carrying passengers around a city, the hormones are the passengers, and while they are in the taxi, they cannot get out and do their jobs. Only the hormones that are free, not bound to SHBG, can enter your cells and have an effect.

When SHBG levels are high, more hormones are bound up and fewer are available to tissues; when SHBG is low, more hormones circulate freely. This means that even if your total testosterone or oestrogen looks perfectly normal, your SHBG level could mean you effectively have too much or too little active hormone reaching your cells. Your body adjusts SHBG production in response to a variety of signals, making it a sensitive barometer of metabolic health.

A number of factors can influence your SHBG levels. Oestrogen tends to increase SHBG production, which is why women generally have higher levels than men and why hormonal contraceptives can raise SHBG significantly. Conversely, carrying excess body weight, particularly around the abdomen, tends to lower SHBG, as does insulin resistance. Thyroid function also plays a part, an overactive thyroid can push SHBG up, while an underactive thyroid can bring it down. Age, liver health, and certain medications further contribute to the picture. If your SHBG level is outside the expected range, your GP can look at it alongside your free and total sex hormone levels to assess whether your hormonal balance is working well for you.