Thyroid Cytomel (liothyronine) and perchlorate suppression studies are nuclear laboratory tests performed to determine thyroid function. Cytomel suppression is performed to assist in diagnosing goiter disease and borderline hyperthyroidism. It involves the administration of triiodothyronine followed by scanning for 123I or 131I uptake, revealing the effect of suppression or the absence of suppression. Normally, uptake decreases more than 60 percent when compared with an initial scan. Hyperthyroidism evaluated in this test is caused by Graves' disease (toxic goiter), an autoimmune disease causing antibodies to bind to thyroid cells, resulting in glandular hypertrophy and the secretion of thyroid hormone. It can also be caused by Plummer's disease (multinodular goiter), resulting in an increased gland size and progressively developing hyperthyroidism or a solitary hyperfunctional adenoma (toxic adenoma) that is multinodular and produces hyperthyroidism. This test has generally been replaced by the TRH stimulation test.

Perchlorate suppression is performed to assist in diagnosing organification defects responsible for hypothyroidism, a term used to indicate a hypometabolic state caused by deficient thyroid secretions. The test involves measurement of thyroid gland activity after scanning for 131I uptake and subsequent admission of potassium perchlorate. Abnormal findings reveal a fall in uptake in 2 hours when compared with uptake after the perchlorate intake. The counts and timing of the uptake are displayed on a graph from which thyroid activity is calculated.⁷⁸

• Foods and medications containing iodine taken before the study

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Indications

Cytomel Suppression

• Evaluation of the effect of T3 on thyroid uptake of the radionuclide in the diagnosis of thyroid function:
  • Uptake is increased initially and decreases after Cytomel in thyroiditis, deficient iodine, or other hormone abnormality. Decrease can be more than half of the baseline level.
  • Uptake is increased initially, and no change is demonstrated after Cytomel in Graves' disease, Plummer's disease, and toxic adenoma. Suppression is absent in borderline hyperthyroidism.
• Diagnosis of thyroid pathology that causes changes in uptake after Cytomel and subsequent suppression:
  • Suppression is absent if nodules are present.
  • Slight suppression is absent if thyroid malignancy is present.
• Absence of changes in uptake after thyroid therapy that causes destruction of tissue

Perchlorate Suppression

• Diagnosis of thyroid disorders that reveal changes in the organification process that causes decreases in uptake of the radionuclide:
  • Congenital hypothyroidism in infants with a decrease of at least 15 percent in uptake after perchlorate indicates a congenital enzyme deficiency (peroxidase) within the gland as perchlorate removes unbound iodine and prevents further uptake.
  • Decrease in uptake after perchlorate in Hashimoto's thyroiditis is commonly found in autoimmune diseases such as lupus erythematosus and rheumatoid arthritis, as well as pernicious anemia and Addison's disease, because iodine is prevented from being concentrated by the thyroid and unbound iodide is prevented from being released.

Contraindications

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

[Section Outline]

• Indications
  • Cytomel Suppression
  • Perchlorate Suppression
• Contraindications

Nursing Care Before the Procedure

Client preparation is the same as for any nuclear scan study (see section under "Brain Scanning").

Cytomel Suppression. Instruct the client to take ordered 25 mg of Cytomel every 8 hours for 5 to 10 days, explain the schedule, and ask the client to return to the department for the scanning.

• Inform the client that foods and medications containing iodine should be restricted before the test.
• Explain that a radionuclide is administered IV and that some discomfort will be experienced at that time but that this is the only discomfort experienced during the test.
• History should include previous thyroid tests, allergies to iodine, medication regimen, and other therapy.

Perchlorate Suppression. Preparation is the same as for Cytomel suppression except that food and fluids are withheld 8 to 12 hours before the test and foods and medications containing iodine are withheld for at least 1 week before the test. The client can be given a list of foods and medications to avoid. Inform the client that the radionuclide is administered orally instead of IV and that the perchlorate is given orally during the test.

The procedure for radioactive iodine uptake is found in the preceding section.

Cytomel Suppression. The client comes to the nuclear medicine department 1 day before the last dose of Cytomel. Scanning is performed at that time and residual uptake is recorded. The radionuclide 123I or 131I is administered, and scanning is then performed 2 and 6 hours later.

The client is requested to return to the department after the last dose of Cytomel and an additional uptake scan is performed. Uptakes are compared, and increases or other changes are determined, based on the dose of the radionuclide, the timing of the medications, and scanning.

Perchlorate Suppression. Scanning is performed to determine whether any residual radionuclide is present. The radionuclide is administered orally, and scanning for uptake is performed in 1 and 2 hours. After the 2-hour measurement, 1 g of perchlorate is given orally, as this dose is trapped by the thyroid and displaces the iodide that has not been organified. Uptakes obtained by scanning are then performed every 15 minutes for 90 minutes. Additional scanning can be performed every 30 minutes for the next 2 hours. Uptakes after the perchlorate and those performed 2 hours before the perchlorate are compared and results graphed, based on time of uptakes, amount of the radionuclide concentrated in the thyroid gland, or absence of it in the gland.

Nursing Care After the Procedure

Care and assessment after the procedures are the same as for any nuclear scan study (see section under "Brain Scanning").

• Resume normal dietary intake and medication regimen.
• Provide a written schedule for return to the department for scanning and test completion.