TSH ELISA

Thyroid-stimulating hormone (TSH or thyrotropin) is a
glycopolipeptide hormone synthesized and secreted
by the anterior pituitary gland which regulates the
endocrine function of the thyroid gland.
TSH consists of two subunits, the alpha and the beta
subunit. The  subunit is identical to that of human
chorionic gonadotropin (HCG), luteinising hormone
(LH), follicle-stimulating hormone (FSH). The β (beta)
subunit is unique to TSH, and therefore determines
its function.
TSH production is controlled by a Thyrotropin
Releasing Hormone, (TRH), which is manufactured in
the hypothalamus and transported to the pituitary
gland, where it increases TSH production and
release. Somatostatin is also produced by the
hypothalamus, and has an opposite effect on the
pituitary production of TSH, decreasing or inhibiting
its release.
TSH stimulates the thyroid gland to secrete the
hormones thyroxine (T4) and triiodothyronine (T3).
Release of TSH is regulated by the circulating free
fraction of thyroid hormones in the blood. TSH levels
are depressed when peripheral concentrations of the
free fraction of thyroid hormones are high.
Conversely, TSH levels are high when peripheral
concentrations of thyroid hormones are low. This
effect creates a regulatory negative feedback loop.
TSH levels are tested in the blood of patients
suspected of suffering from excess (hyperthyroidism),
or deficiency (hypothyroidism) of thyroid hormone.
Generally, a normal range for TSH is between 0.3
and 3.0 mIU/mL, but the interpretation depends also
on what the blood levels of thyroid hormones (T3 and
T4) are.
Higher than normal levels of TSH combined with high
levels of thyroid hormone (T3 and T4) may indicate
dysfunction of the hypothalamus and pituitary gland.
In these case, a high TSH is often produced by a
benign
tumour of the pituitary (adenoma). Conversely, low
levels of TSH, while blood levels of T3 and T4 are
also low, indicates abnormally low function of the
pituitary, known as hypopituitarism.
On the other hand, due to the negative feedback
described above, abnormally high levels of Thyroid
hormone, due to overproduction in the thyroid, results
in low TSH levels. This occurs in diseases such as
hyperthyroidism or Grave’s disease. Conversely, an
underproduction of T3 and T4 caused by diseases
such as congenital hypothyroidism (cretinism),
hypothyroidism or thyroid hormone resistance, gives
rise to an increase in the measured TSH.
Clearly both TSH and T3 and T4 should be measured
to ascertain where a specific thyroid disfunction is
caused by primary pituitary or by a primary thyroid
disease.