How low?besides SHBG what other levels were checked, low levels can indicate a few other hormonal issues such as diabetes, hypothyriodism and lipids if male can be a marker of testicular cancer or female ovarian cancer.
An unbalanced state of hormonal changes can raise or lower SHBG, giving false indication of how other circulating hormones are responding , stress, low carb diets,exogenous hormones can change SHBG.
Did you pull this or your doctor?