Sex hormone binding globulin (SHBG) is a protein produced primarily by the liver that binds to certain hormones in the bloodstream,  including testosterone, dihydrotestosterone (DHT), and estradiol. These hormones are essential for the development of sexual and reproductive tissues in both males and females, and SHBG helps regulate the levels of these hormones in the body.
What is SHBG and What Does it Do?
SHBG is an essential protein that binds to sex hormones in the bloodstream, regulating their distribution and availability to the body's cells. These hormones play a crucial role in the development of sexual and reproductive tissues, but too much or too little can lead to various health issues. SHBG binds three hormones, including testosterone, DHT, and estradiol, and carries them throughout the bloodstream. [2,3,4]
When SHBG levels are low in the body, there are more unbound sex hormones available for use by the cells. Conversely, when SHBG levels are high, fewer free sex hormones are available to the body. The levels of SHBG in the body can change throughout a person's lifetime, and many factors can influence the normal levels of SHBG.
What are Normal SHBG Levels?
Normal SHBG levels vary by gender and age. In males, the normal range is between 10 to 57 nanomoles per liter (nmol/L), while in females, the normal range is between 18 to 144 nmol/L. Men typically have lower SHBG levels than women, and a man's SHBG level will usually increase with age as his testosterone levels drop. Pregnancy usually raises SHBG levels, and they typically return to normal after childbirth. 
Please keep in mind that the normal range values may vary from lab to lab depending on where you have this test performed.
Why is SHBG Testing Important?
SHBG testing is essential in diagnosing various conditions and diseases related to abnormal testosterone levels. The test measures the level of SHBG in the blood and can help diagnose androgen deficiency, hypogonadism, and other health issues related to testosterone levels.
Your healthcare provider may also order total and free testosterone blood levels along with the SHBG test to confirm an androgen deficiency. The test is simple and involves taking a blood sample from a vein in your arm or hand.
Factors that Affect SHBG Levels
Many factors can influence SHBG levels in the body, including sex, age, weight, genetics, and metabolism. Low levels of SHBG can be related to obesity, type 2 diabetes, hypothyroidism, acromegaly, or too much growth hormone, causing body tissues to grow larger over time. High levels of SHBG can be related to hepatitis, hyperthyroidism, HIV, and anticonvulsants, or medicines used to treat seizures. [6.7]
Opioids used for pain relief, medicines that affect the central nervous system, and recreational drug use can all affect SHBG test results. Having an eating disorder or engaging in excessive, strenuous exercise can also affect test results.
How to Increase SHBG Levels
Treatment of low SHBG levels depends on the cause, and any underlying conditions will have to be addressed. Regular exercise, drinking coffee, taking certain oral contraceptives, increasing fiber, decreasing sugar in your diet, losing weight, and taking certain supplements have been found to increase SHBG levels. 
A randomized clinical trial of sedentary men aged 40 to 75 found that a year-long program of moderate aerobic exercise increased SHBG and DHT.  A large-scale trial in a select population found evidence that SHBG can be increased through exercise in postmenopausal women who were mostly overweight and previously sedentary.
Research on women over age 60 indicates that having two or more cups of regular caffeinated coffee a day is associated with higher SHBG concentrations.  According to a meta-analysis of women with PCOS, SHBG levels increased after three months to one year of treatment with certain combined oral contraceptives.
A 2000 study involving men between the ages of 40 and 70 found that fiber intake increased SHBG levels, while protein intake lowered levels.  However, researchers of this study note their results differ from findings in previous studies. A recent study of postmenopausal women looked at the links between diet and SHBG and indicated that low glycemic load or glycemic index diets with low sugar and high fiber could be associated with higher SHBG concentrations.  More studies are needed to investigate this relationship.
Other research shows that when children who are obese lose weight, SHBG levels can rise significantly. 
Many herbal and dietary supplements claim to help lower SHBG levels to boost testosterone, but it's difficult to know for sure. Supplements aren't regulated by the FDA, so manufacturers are free to make claims that may not be true. Some ingredients have shown promise, [14,15] but often these aren't in the correct doses when compared against what has been trialled.
1 - https://www.ncbi.nlm.nih.gov/gtr/genes/6462/
2 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9240586/
3 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6996888/
4 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5413109/
5 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2648802/
6 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8571812/
7 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8475196/
8 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7663738/
9 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3040039/
10 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3012180/
11 - https://pubmed.ncbi.nlm.nih.gov/10634401/
12 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9051085/
13 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7761898/
14 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC8006238/
15 - https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5612985/