Hormone menubar
sex_hormone_binding_carriers
Sex Hormone-Binding Carrier Proteins
• Sex Hormone Binding Globulin (SHBG)
• Corticosteroid Binding Globulin (CBG)
(sex hormone binding
globulin)
Aka. TeBG (TESTOSTERONE -estrogen
Binding Globulin)
SHBG
binds to
estrogen
and
TESTOSTERONE
in
the blood (making them less
bioavailable)
- Bioavailability of ESTRADIOL or
TESTOSTERONE
is determined by the level of SHBG -
ESTRADIOL and
TESTOSTERONE travel in the blood either strongly
(but reversibly) bound to SHBG (ESTRADIOL 40%) or loosely bound to serum albumin
(ESTRADIOL 59%).E.g. Only about
1-2% of circulating ESTRADIOL is "free"to
enter a cell and activate its receptor.
SBGH level is delicately balanced by enhancing
and inhibiting factors:
-
SHBG is ▲
increased by:
(makes otherwise unbound hormones LESS
bioavailable)
• Liver disease, alcohol
• Hyperthyroidism -high levels of thyroxine (T3)
• Anorexia
• High levels of
estrogen
- E.g.
use of HRT, oral contraceptives
(metabolized by liver);
• High levels of GROWTH HORMONE
-
SHBG is ▼
reduced by: (makes otherwise
bound hormones MORE bioavailable)
• Higher circulating INSULIN levels - (note
however, that recent evidence reveals that liver's fat production reduces SHBG,
and not any direct effect of
INSULIN) ;
• Obesity -
obesity leads to high insulin levels, which causes a reduction in the liver's
productionand circulating levels of
SHBG (see note above); note that obesity also increases
estrogen
production in adipose cells via the
aromatase
enzyme;
Thus, obesity can
promote cancers such as breast and endometrial cancer -as
estrogen
becomes more
bioavailable (free/unbound to SHBG) for its target tissues.
•
Hypothyroidism;
•
Androgens ;
• Corticosteroid-binding
globulin (CBG)
• INSULIN-LIKE GROWTH FACTOR-1( IGF-1)
• Cushing's disease;
• Soy - up to 10%
reduction;
• Nettle root - contains compounds that bind to SHBG; nettle root is also anti -aromatase
and therefore acts against
estrogen
production.
High/Low SHBG Conditions
- Low SHBG
(making hormones
MORE bioavailable) -
seen in polycystic ovary syndrome, diabetes,
hypothyroidism.
- High SHBG
(making hormones
LESS bioavailable) - seen in
pregnancy ,
hyperthyroidism , anorexia nervosa. Recent
research links high SHBG levels with breast and testicular cancer.
SHBG production sites
- SHBG is produced mainly in
the liver cells and released into the bloodstream -
other production
sites are the brain, uterus, placenta and
testes (where it is sometimes called
androgen -binding protein).
(Corticosteroid-binding
Globulin)
CBG
(a.k.a. transcortin)
binds to
PROGESTERONE ,
CORTISOL (and other
corticosteroids) in the blood
- Bioavailability of
PROGESTERONE ,
CORTISOL (and other corticosteroids)
is determined by level of CBG
- PROGESTERONE is bound by CBG (~18%),
albumin
(~80%), SHBG (0.6%)
and ~2% exists in the free state
Clinical
Reproductive medicine and surgery, 2007
CBG level is balanced by enhancing and inhibiting
factors:
-
CBG is ▲
increased by:
(makes otherwise unbound hormones LESS
bioavailable)
• Estrogens / Pregnancy
-
CBG is ▼
reduced by:
(makes otherwise bound hormones MORE
bioavailable)
• Cirrhosis (chronic liver disease);
• Alcohol
CBG production
- CBG is produced by
the liver
