TSH study is a nuclear laboratory test performed to measure thyroid gland response to the administration of a dose of TSH. The test involves the administration of TSH intramuscularly (IM) followed by an IV injection of iodide I 123 and scanning to determine the effect of TSH on thyroid gland uptake of the radionuclide.⁷⁷ It is performed in combination with the radioactive iodine uptake study to differentiate between primary and secondary hypothyroidism. TSH, also known as thyrotropin, is produced by the basophil cells of the adenohypophysis in response to stimulation by its hypothalamic releasing factor, thyrotropin-releasing hormone (TRH). Primary hypothyroidism that results from thyroid pathology does not respond to the TSH dose, whereas secondary hypothyroidism, resulting from an abnormally reduced stimulation by the pituitary gland secreting the TSH, causes a positive response to the TSH dose. Thyroid and pituitary glands therefore can both be evaluated with this test.

• Recent radionuclide scanning, which can affect uptake
• Foods or medications containing iodine, which can decrease uptake of iodine
• Aspirin, heparin, dopamine, or corticosteroids, which can decrease levels of TSH

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Indications

• Evaluating thyroid function to differentiate between primary and secondary hypothyroidism:
  • TSH level decreased in secondary hypothyroidism (hypothalamic or pituitary gland source) as revealed by near 0 levels with a low T4 level because TSH cannot be secreted
  • TSH level increased in primary hypothyroidism (Hashimoto's thyroiditis, congenital disorders, thyroidectomy, radioactive therapy) as revealed by TSH level greater than three times the normal range and a decreased T4 level, causing a compensatory rise in TSH
  • Diminished uptake of the radionuclide in RAIU study at all intervals in hypothyroidism
• Monitoring response to thyroid hormone therapy

Contraindications

• Pregnancy, unless the benefits of performing the procedure greatly outweigh the risks to the fetus

[Section Outline]

• Indications
• Contraindications

Nursing Care Before the Procedure

Client preparation is the same as for any nuclear laboratory test (see section under "Total Blood Volume Study") and nuclear scan study (see section under "Brain Scanning").

• Inform the client of the scanning schedule and explain that the hormone is administered IM by the physician twice during the study.
• Inform the client that a counterscanner is placed over the region of the thyroid gland each time in the series that the test is performed.

The client is given 10 units of TSH by IM injection by the physician and placed on the examining table in a supine position with the head slightly hyperextended on a small pillow. Counts are taken over the thyroid gland, followed by the IM injection of another 10 units of TSH and the oral administration of the radionuclide for the RAIU scan. An uptake and scan are performed in 2 to 6 hours and again 24 hours later. Refer to the radioactive iodine uptake scan procedure (see preceding section) and thyroxine laboratory test (see Chapter 5 - Blood Chemistry, "Thyroxine" section) for further information about this test.

Nursing Care After the Procedure

Care and assessment after the procedure are the same as for any nuclear scan study (see section under "Brain Scanning") and venipuncture for injection of the radionuclide (see Appendix I).