Sex Hormone Binding Globulin (SHBG)

Function

SHBG is a protein produced by the liver that binds tightly to three sex hormones: testosterone, dihydrotestosterone (DHT), and estradiol. While bound to SHBG, these hormones are 'inactive' and cannot be used by the body's tissues. Therefore, SHBG acts as a regulator, controlling the amount of 'free' or bioavailable hormones circulating in the blood.

Why it is Ordered

This test is rarely ordered alone; it is usually part of a 'Free Testosterone Index' or used to investigate symptoms of hormone imbalance. In women, it is used to evaluate Poly cystic Ovary Syndrome (PCOS) or hirsutism (excess hair growth). In men, it helps explain why a patient might have symptoms of low testosterone despite having 'normal' total testosterone levels.

Associated Conditions

• PCOS: Often associated with low SHBG and high free testosterone.
• Hyperthyroidism: Frequently causes high SHBG levels.
• Liver Cirrhosis: Can cause significant elevations in SHBG.
• Insulin Resistance: High insulin levels typically suppress SHBG production.

Clinical Implications

A high SHBG level means more testosterone is 'locked up' and unavailable, which can lead to symptoms of androgen deficiency in men. Conversely, low SHBG means more free testosterone is available, which in women can lead to acne, weight gain, and menstrual irregularities.