SHBG

What it is

SHBG (Sex Hormone-Binding Globulin) is a protein produced by the liver. Its role is to transport sex hormones in the bloodstream — mainly testosterone and oestradiol — and keep them in reserve in an inactive form.

Hormones bound to SHBG cannot act on target cells. Only the free (unbound) fraction is biologically active. SHBG therefore acts as a regulator: the higher it is, the less active the hormones; the lower it is, the more available they are.

Why it matters in PCOS

In PCOS, SHBG is often lower than normal. This reduction has a direct consequence: even if total testosterone is within the reference range, the free (active) fraction can be elevated, contributing to clinical hyperandrogenism — acne, hirsutism, hair loss.

This is why a complete PCOS workup often includes SHBG alongside total testosterone, to calculate the Free Androgen Index (FAI) (FAI = total testosterone ÷ SHBG × 100) or estimate free testosterone.

What lowers SHBG

Several factors reduce the liver's production of SHBG, thereby increasing androgen bioavailability:

• Excess insulin (hyperinsulinaemia) — common in PCOS — is one of the best-documented factors. Insulin resistance lowers SHBG, which amplifies hyperandrogenism
• Androgens themselves — excess testosterone lowers SHBG (a self-amplifying loop)
• Hypothyroidism
• Obesity (through hyperinsulinaemia)

Conversely, oestrogens (particularly in combined hormonal contraceptives) and thyroid hormones raise SHBG — which explains why some contraceptives can improve androgenic symptoms.

How SHBG is used in practice

SHBG alone is not a diagnostic criterion for PCOS. It is interpreted alongside total testosterone, and sometimes androstenedione and DHEAS, to give a complete picture of androgenic status. The Free Androgen Index can be more informative than total testosterone alone.

Key takeaways

• A protein that transports sex hormones and keeps them inactive
• Often low in PCOS — which increases active androgens
• Lowered by hyperinsulinaemia and androgens themselves
• Interpreted alongside testosterone to estimate the free fraction
• Can improve with better insulin sensitivity