Can high SHBG with low free testosterone amplify low-androgen signaling in postmenopausal women?

Clinical context and signaling

• In postmenopausal women, evaluating and treating low-androgen signaling is highly context-dependent and primarily indicated for Hypoactive Sexual Desire Disorder (HSDD).
• High circulating sex hormone-binding globulin (SHBG) in combination with low free testosterone restricts the ligand pool available for passive cellular diffusion, which directly exacerbates and amplifies low-androgen signaling at the tissue level.

Molecular and mechanistic pathways

• High-Affinity Equilibrium: SHBG regulates steroid availability by binding testosterone with high affinity ($K_a \approx 1 \times 10^9$ to $2 \times 10^9 \text{ M}^{-1}$, with a $K_d \approx 3 \text{ to } 4 \text{ nM}$ for estradiol), dynamically reducing the unbound, bioavailable fraction.
• Megalin-Mediated Endocytosis: Although elevated SHBG suppresses the classic free hormone pool, certain tissues express Megalin, a clathrin-dependent endocytic receptor. Megalin binds and internalizes circulating SHBG-androgen complexes, delivering them directly inside target cells to sustain intracellular androgen receptor activation even under low-free-hormone conditions.

Bottom line

• High SHBG and low free testosterone act in concert to reduce the bioavailable hormone fraction required for passive receptor activation, amplifying low-androgen signaling in postmenopausal women, though cell-surface endocytic pathways via Megalin offer a localized mechanism to modulate tissue-specific uptake.