Hormone-binding globulins and total vs free hormone levels (CBG, SHBG, TBG)
Core concept
- Only the free hormone is biologically active - the bound fraction is a circulating reservoir
- Free hormone hypothesis: the axis regulates the free level
- A change in binding protein alters total hormone but the feedback loop restores free hormone within days
- -> total level changes, TSH/ACTH/LH stay normal, patient is euthyroid/eucortisolaemic/eugonadal
- -> The commonest cause of an abnormal total hormone with a normal axis is a change in binding globulin, not disease
| Binding protein | Carries | Bound fraction |
|---|---|---|
| TBG (+ transthyretin, albumin) | T4, T3 | ~99.97% of T4 |
| CBG (transcortin) (+ albumin) | Cortisol, progesterone, some aldosterone | ~90-95% of cortisol |
| SHBG (+ albumin) | Testosterone, oestradiol, DHT | ~98% of testosterone |
| Vitamin D binding protein | 25(OH)D, 1,25(OH)2D | ~99% |
| IGFBP-3 + ALS | IGF-1 | ~99% |
3 more sections, plus exam facts
Premium unlocks every note across every specialty, and the full exam fact library behind it.
Get premium access