Type of test Blood

 Normal findings

Baseline TSH: < 10 μU/mL

Stimulated TSH: more than double the baseline

Test explanation and related physiology

 The TRH stimulation test assesses the anterior pituitary gland via its secretion of TSH in response to an IV injection of TRH. After the TRH injection, the normally functioning pituitary gland should secrete TSH. In hyperthyroidism, either a slight or no increase in the TSH level is seen because pituitary TSH production is suppressed by the direct effect of excess circulating T4 and T3 on the pituitary gland. A normal result is considered reliable evidence for excluding the diagnosis of thyrotoxicosis. Since the development of very sensitive radioimmunoassay for TSH, the TRH stimulation test is no longer required to diagnose hyperthyroidism. However, it still has a role in the evaluation of pituitary deficiency.

In addition to assessing the responsiveness of the anterior pituitary gland, this test aids in the detection of primary, secondary, and tertiary hypothyroidism. In primary hypothyroidism (thyroid gland failure), the increase in the TSH level is two or more times the normal result. With secondary hypothyroidism (anterior pituitary failure), no TSH response occurs. Tertiary hypothyroidism (hypothalamic failure) may be diagnosed by a delayed rise in the TSH level. Multiple injections of TRH may be needed to induce the appropriate TSH response in this case.

The TRH test also may be useful in differentiating primary depression from manic-depressive psychiatric illness and from secondary types of depression. In primary depression, whereas the TSH response is blunted in most patients, patients with other types of depression have a normal TRH-induced TSH response.

Interfering factors

• Pregnancy may increase the TSH response to TRH.

* Drugs that may modify the TSH response include antithyroid drugs, aspirin, corticosteroids, estrogens, levodopa, and T4 .

Procedure and patient care

• See inside front cover for Routine Blood Testing.

 • Fasting: no

• Blood tube commonly used: verify with laboratory

* Instruct the patient to discontinue thyroid preparations for 3 to 4 weeks before the TRH test, if indicated.

• Assess the patient for medications currently being taken.

• Administer a prescribed IV bolus of TRH.

• Obtain venous blood samples at intervals, and measure for TSH levels.

• Indicate on the laboratory slip if the patient is pregnant.

Abnormal findings

- Hyperthyroidism

- Hypothyroidism

- Psychiatric primary depression

- Acute starvation

- Old age (especially in men)

- Pregnancy

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