It is a 210 amino acid, two chain glycoprotein (22% sugar), MW 30000.
THYROID STIMULATING HORMONE (TSH,
THYROTROPIN)
It
is a 210 amino acid, two chain glycoprotein (22% sugar), MW 30000.
Physiological Function
TSH stimulates thyroid to synthesize and secrete thyroxine (T4)
and triiodothyronine (T3). Its actions are:
· Induces hyperplasia and hypertrophy of thyroid
follicles and increases blood supply to the gland.
· Promotes trapping of iodide by thyroid.
· Promotes organification of trapped iodine and
its incorporation into T3 and T4 by increasing peroxidase
activity.
· Enhances endocytotic uptake of thyroid colloid
by the follicular cells and proteolysis of thyroglobulin to release more of T3
and T4. This action starts within minutes of TSH administration.
The
TSH receptor present on thyroid cells is a G protein coupled receptor which
utilizes the adenylyl cyclase cAMP transducer mechanism to produce its effects.
In human thyroid cells high concentration of TSH also induces PIP2
hydrolysis. The resulting increase in cytosolic Ca2+ and protein kinase C activation
may also mediate TSH actions.
Regulation Of Secretion
Synthesis and release
of TSH by pituitary is
controlled by hypothalamus through TRH. The negative feed back inhibiting TSH
secretion is provided by the thyroid hormones which act primarily at the level
of the pituitary, but also in the hypothalamus. T3 has been shown to
reduce TRH receptors on thyrotropes.
Pathological Involvement
Only few cases of hypo
or hyperthyroidism are
due to inappropriate TSH secretion. In majority of cases of myxoedema TSH levels
are markedly elevated because of deficient feedback inhibition. Graves’ disease
is due to an immunoglobulin of the IgG class which attaches to the thyroid
cells and stimulates them in the same way as TSH. Consequently, TSH levels are
low. Contrary to earlier belief, TSH is not responsible for exophthalmos seen
in Graves’ disease because TSH levels are low.
Use
Thyrotropin
has no therapeutic use. Thyroxine is the drug of choice
even when hypothyroidism is due to TSH deficiency. The diagnostic application
is to differentiate myxoedema due to pituitary dysfunction from primary thyroid
disease.
Related Topics
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