The TSH stimulation test is used to evaluate the thyroid-pituitary-hypothalamic feedback loop. In this test, a purified form of hypothalamic thyrotropin-releasing hormone (TRH) is administered IV. Normally, TRH stimulates the adenohypophysis to release TSH, which, in turn, causes hormonal release from the thyroid gland. A normal response (e.g., elevated TSH levels) indicates that the adenohypophysis is capable of responding to TRH stimulation. If thyroid hormones also are measured as part of the test, elevated levels indicate that the thyroid gland is capable of responding to TSH stimulation.

• TSH levels rise within 15 to 30 minutes of TRH administration, peak at 2.5 to 4 times normal, and return to baseline levels within 2 to 4 hours. Thyroid hormone secretion (e.g., T₃ and T₄), which should be increased by 50 to 75 percent, occurs in 1 to 4 hours.

• Low or undetectable serum TSH levels, hypothyroidism, or hyperthyroidism of unknown etiology or type, or low serum TSH levels combined with hypothyroidism or hyperthyroidism:
  • A normal or delayed TSH response in persons with low baseline TSH levels and signs of hypothyroidism indicates hypothalamic dysfunction or disruption of the hypothalamic-hypophyseal portal circulation and confirms the diagnosis of tertiary hypothyroidism.
  • A decreased or absent TSH response in persons with low baseline TSH levels and signs of hypothyroidism indicates hypopituitarism and confirms the diagnosis of secondary hypothyroidism.
  • A normal or increased TSH response in clients with elevated baseline TSH levels and signs of hypothyroidism, with persistently decreased thyroid gland hormone levels, confirms the diagnosis of primary hypothyroidism.
  • A decreased or absent TSH response in persons with low baseline TSH levels and signs of hyperthyroidism, with persistently elevated thyroid gland hormone levels, indicates that thyroid hormone production is occurring autonomously and confirms the diagnosis of primary hyperthyroidism.

Nursing Care Before the Procedure

Initial client preparation is the same as that for serum determinations of TSH.

• Because several blood samples will be obtained and because the TRH will be administered IV, the client should be informed that an intermittent venous access device (e.g., heparin lock) may be inserted.

The procedure varies somewhat according to the laboratory performing the test. One example of the procedure is described subsequently.

An intermittent venous access device is inserted and a venous sample is obtained and placed in a red-topped tube. The sample is handled gently to avoid hemolysis and sent promptly to the laboratory. The sample should be labeled either with the time drawn or as the baseline sample.

A bolus of TRH is then administered IV through the access device. Additional blood samples are obtained via the access device ½, 1, 2, 3, and 4 hours after administration of the TRH. Each sample is placed in a red-topped tube, labeled, and sent to the laboratory.

Nursing Care After the Procedure

Care and assessment after the procedure are essentially the same as for serum TSH determinations.

• If an intermittent venous access device was inserted for the procedure, remove after completion of the test and apply a pressure bandage to the site.
• Complications and precautions: Note increased levels in relation to other thyroid tests and prepare client for a nuclear scan laboratory study using the iodine 131 (¹³¹I) radionuclide (see Chapter 20 - Nuclear Scan and Laboratory Studies).