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Introduction

  1. Explain test purpose and blood specimen collection procedure. To understand the thyroid function tests, it is necessary to understand the following basic concepts. The thyroid gland takes iodine from the circulating blood, combines it with the amino acid tyrosine, and converts it to the thyroid hormones thyroxine (T4) and triiodothyronine (T3). Iodine composes about two thirds of the weight of the thyroid hormones. The thyroid gland stores T3 and T4 until they are released into the bloodstream under the influence of TSH from the pituitary gland. Only a small amount of the hormone is not bound to protein. However, it is the free portion of the thyroid hormones that is the true determinant of the thyroid status of the patient.

  2. Assess for signs and symptoms of thyroid disease and note thyroid and iodine medications. Fasting is required for some tests.

  3. A typical thyroid panel may include the following tests:

    1. T3 uptake (T3 U)

    2. Free T4 (FT4)

    3. Total T4

    4. T3 total

    5. Free thyroxine index (FTI, T7)

    6. TSH

  4. The most useful laboratory tests to confirm or exclude hyperthyroidism are total T4, FTI, total T3, and TSH. The most useful tests to detect hypothyroidism are total T4, FTI, and TSH. A thyrotropin-releasing hormone (TRH) stimulation test can be valuable in establishing the thyroid status in some patients with equivocal signs of thyroid dysfunction and borderline laboratory values. It should be kept in mind that values obtained for the assessment of thyroid function can be influenced by factors other than disease, such as age, current illness, binding capacity of serum proteins, and some drugs.

  5. In patients with stable thyroid status, TSH is more sensitive than an FT4 measurement for detecting mild thyroid hormone deficiency or excess. In patients with unstable thyroid status, FT4 is a better indicator of posttreatment status than TSH.

  6. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

  1. Review test results; report and record findings. Modify the nursing care plan as needed. Counsel and monitor appropriately for abnormal thyroid function and disease. Follow-up testing may be required.

  2. Thyroid antibody testing can also be done for diagnosis of autoimmune thyroid disease.

  3. Follow guidelines in Chapter 1 for safe, effective, informed posttest care.