Prior to the availability of sensitive TSH assays,
thyrotropin releasing hormone
Thyrotropin-releasing hormone (TRH) is a hypophysiotropic hormone produced by neurons in the hypothalamus that stimulates the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary.
TRH has been used clinical ...
or TRH stimulation tests were relied upon for confirming and assessing the degree of suppression
in suspected
hyperthyroidism
Hyperthyroidism is the condition that occurs due to excessive production of thyroid hormones by the thyroid gland. Thyrotoxicosis is the condition that occurs due to excessive thyroid hormone of any cause and therefore includes hyperthyroidis ...
. Typically, this stimulation test involves determining basal
TSH levels and levels 15 to 30 minutes after an intravenous bolus of TRH. Normally,
TSH would rise into the concentration range measurable with less sensitive TSH assays.
Third generation TSH assays do not have this
limitation and thus TRH stimulation is generally not required when third generation TSH
assays are used to assess degree of suppression.
Differential diagnosis use
TRH-stimulation testing however
continues to be useful for the differential diagnosis of secondary (pituitary disorder) and tertiary (hypothalamic disorder) hypothyroidism. Patients with these conditions
appear to have physiologically inactive TSH in their circulation that is recognized by
TSH assays to a degree such that they may yield misleading, "
euthyroid
The thyroid, or thyroid gland, is an endocrine gland in vertebrates. In humans it is in the neck and consists of two connected lobes. The lower two thirds of the lobes are connected by a thin band of tissue called the thyroid isthmus. The thy ...
" TSH
results. Use and Interpretation:
• Helpful in diagnosis in patients with confusing TFTs. In primary hyperthyroidism
TSH are low and TRH administration induces little or no change in TSH levels
• In hypothyroidism due to end
organ failure
Organ dysfunction is a condition where an organ does not perform its expected function. Organ failure is organ dysfunction to such a degree that normal homeostasis cannot be maintained without external clinical intervention.
It is not a diagnosis ...
, administration of TRH produces a
prompt increase in TSH
• In hypothyroidism due to pituitary disease (secondary hypothyroidism) administration of TRH does not produce
an increase in TSH
• In hypothyroidism due to hypothalamic disease (tertiary hypothyroidism), administration of TRH produces a
delayed (60–120 minutes, rather than 15–30 minutes) increase in TSH
Process and interpretation
The TRH test involves administration of a small amount of TRH intravenously, following which levels of TSH will be measured at several subsequent time points using samples of blood taken from a peripheral vein.
The test is used in the differential diagnosis of secondary and tertiary hypothyroidism. First, blood is drawn and a baseline TSH level is measured. Then, TRH is administered via a vein. After 30 minutes blood is drawn again and the levels of TSH are measured and compared to the baseline. Some authors recommend additional blood sampling at 15 minutes. In children, late blood sampling at 60 to 120 minutes is necessary.
An increase in the serum TSH level following TRH administration means that the cause of the hypothyroidism is in the
hypothalamus
The hypothalamus () is a part of the brain that contains a number of small nuclei with a variety of functions. One of the most important functions is to link the nervous system to the endocrine system via the pituitary gland. The hypothalamu ...
(tertiary hypothyroidism), i.e. the hypothalamus is not producing TRH. Therefore, when TRH is given exogenously, TSH levels increase. If the increase in serum TSH level following TRH administration is absent or very slight, then the cause of the hypothyroidism is in the
anterior pituitary gland
A major organ of the endocrine system, the anterior pituitary (also called the adenohypophysis or pars anterior) is the glandular, anterior lobe that together with the posterior lobe (posterior pituitary, or the neurohypophysis) makes up the p ...
, i.e. the pituitary is not secreting TSH. Therefore, even when TRH is given exogenously, TSH levels do not rise as the pituitary is diseased.
Side effects and risks
TRH may cause nausea, vomiting and some patients experience an urge to urinate.
Rarely, TRH may cause blood vessel constriction leading to
hemorrhage
Bleeding, hemorrhage, haemorrhage or blood loss, is blood escaping from the circulatory system from damaged blood vessels. Bleeding can occur internally, or externally either through a natural opening such as the mouth, nose, ear, urethra, v ...
in patients with pre-existing
pituitary tumors
Pituitary adenomas are tumors that occur in the pituitary gland. Most pituitary tumors are benign, approximately 35% are invasive and just 0.1% to 0.2% are carcinomas.
See also
*
Thyrotropin-releasing hormone
Thyrotropin-releasing hormone (TRH) is a hypophysiotropic hormone produced by neurons in the hypothalamus that stimulates the release of thyroid-stimulating hormone (TSH) and prolactin from the anterior pituitary.
TRH has been used clinicall ...