TSH levels are increased in hypothyroidism when the hypothalamic-pituitary axis is intact, but are decreased in hyperthyroidism due to end-organ overactivity. Thyrotropin-releasing hormone (TRH) normally results in release of TSH from the anterior pituitary. Injection of TRH can be used to assess the release of TSH from the anterior pituitary.
Patient Preparation:
500 micrograms of TRH (Thypinone) are administered intravenously by rapid injection. The test should be done in the morning due to cycle of TSH release.
Specimen Collection:
Specimens are collected immediately prior to injection of the TRH, at 30 minutes after injection and at 60 minutes after. Some protocols also have samples drawn at 15 minutes and 45 minutes after injection.
Testing:
All of the samples are analyzed for TSH. The sample taken prior to TRH injection is tested for thyroxine (T4).
|
Interpretation |
TSH before TRH |
Thyroxine before TRH |
TSH 30 minutes after TRH |
TSH 60 minutes after TRH |
|
normal |
normal |
normal |
peak level, increased 5-10 times baseline |
returning to baseline |
|
primary hypothyroidism |
increased |
decreased |
peak level, marked (> 10 times) increase |
returning to baseline |
|
"hypothalamic" hypothyroidism |
normal or low |
decreased |
increased over baseline |
does not return to baseline, greater than at 30 minutes |
|
hyperthyroidism |
normal or low |
increased |
no or minimal increase |
no or minimal increase |
|
pituitary TSH deficiency |
normal or low |
decreased |
no increase |
no increase |
|
pregnancy |
normal |
normal |
increased response |
return to baseline |
|
other conditions (see below) |
|
|
less than normal increase |
|
Other conditions with decreased TSH response to TRH include:
(1) Cushing's syndrome
(1) systemic illness
(2) beta blocker therapy
(3) depression
(4) corticosteroid therapy
(5) dopamine therapy
(6) advancing age in men
Limitations:
• There is less need today for test due to availability of high-sensitivity TSH assays.
Specialty: Endocrinology, Clinical Laboratory
ICD-10: ,
