Definition
- A glycoprotein, synthesized in the liver, which binds testosterone and 5-dihydrotestosterone with high affinity, and estradiol with a somewhat lower affinity. SHBG typically circulates at higher concentrations in women than in men, due to the higher ratio of estrogens to androgens in women. Administration of androgens tends to be associated with decreased SHBG levels. Because variations in the carrier protein levels may affect the concentration of testosterone in circulation, SHBG levels are commonly measured as a supplement to total testosterone determinations. The "free androgen index " � (FAI), calculated as the ratio of total testosterone to SHBG, has proved to be a useful indicator of abnormal androgen status in conditions such as hirsutism.
- Normal range: see Table 16.74.
� �
TABLE 16 " �74Normal Ranges of Sex Hormone " �Binding GlobulinView LargeTABLE 16 " �74Normal Ranges of Sex Hormone " �Binding Globulin Group Central 95% (nmol/L) Median (nmol/L) Male 13 " �71 32 Female (nonpregnant) 18 " �114 51
Use
- Diagnosis and follow-up of women with symptoms or signs of androgen excess (e.g., polycystic ovarian syndrome and idiopathic hirsutism)
- As an adjunct in monitoring sex steroid and antiandrogen therapy
- As an adjunct in the diagnosis of disorders of puberty
- As an adjunct in the diagnosis and follow-up of anorexia nervosa
Interpretation
Increased In
- Hyperthyroidism
- Hepatic cirrhosis
- Pregnancy
- Drugs: estrogens (e.g., certain oral contraceptives, phenytoin [hepatic enzyme induction])
- Use of dexamethasone in the treatment of women with hyperandrogenic hirsutism
Decreased In
- Hirsutism
- Acne vulgaris
- Polycystic ovary syndrome
- Hypothyroidism
- Acromegaly
- Cushing disease
- Hyperprolactinemia
Limitations
- SHBG can be increased with age, hyperestrogen states, marked weight loss and chronic exercise, HIV infection, cirrhosis.
- Decreased levels may also be due to obesity and protein losing nephropathies.