Fig.1 - Thyroid Hormone Production
(↓TRH release) ↓
TRH in the brain
Iodide Iodide Iodine
93% T4 (inactive) + 7% T3 (active) Cellular mRNA
TGB ↑Cell metabolism
Liver 60% Gastrointestinal sulfatase activity
20% rT3 20% T3S & T3AC
(irreversible inactive) inactive
TRH (Thyrotropin-Releasing Hormone) - is produced in the hypothalamus (in the brain) and stimulates
the pituitary (in the brain) to release TSH. Once T4 reaches
normal levels it stops the release of TRH.
TSH (Thyroid Stimulating Hormone) - when stimulated by TRH the pituitary releases TSH which
stimulates the Thyroid to release T4.
TPO (Thyroid Peroxidase) - enzyme is involved in the production of T4 & T3 thyroid hormones.
T4 (Thyroxine) - is the major thyroid hormone product (93%) although it is inactive and needs to be
transported by TBG to the Liver where it is converted in to T3.
T3 (Triiodothyronine) – is the active thyroid hormone and attaches to receptors on nearly every tissue in
the body (brain, bone, muscle, etc.) this is responsible for the basal metabolic rate (BMR),
cellular energy (ATP) production, protein synthesis, Only about 7% of T3 is produced directly by the thyroid,
the Liver (glucuronidation & sulfation) converts 60% of the T4 made by the thyroid into T3.
TGB - Thyroid Binding Globulin - transports T4 to the Liver, and T3 to the target cells in the tissues
throughout the body. T4 & T3 bound to TGB is active until released as
FT4 & FT3)
fT4 – Free T4 is T4 that is not bound by TGB which is inactive and only about 0.03%
fT3 – Free T3 is T3 that is not bound by TGB which is active and about 0.3%
T3S (T3 Sulfate) & T3AC (Triiodothyroacetic acid) – are converted into T3 by an enzyme
(Gastrointestinal sulfatase produced by
rT3 - Reverse T3 – is irreversible inactive