Endocrine Laboratory Pearls:
Thyroid Tests

Physicians are comfortable ordering and interpreting familiar laboratory tests, but they might not recognize some of the tests' limitations, resulting in over-utilization or in misguided confidence in the validity of the results.

In addition, less familiar laboratory tests that could improve patient care or reduce costs might be ordered too infrequently because of uncertainty about testing indications or result interpretation.

Tests for free thyroid hormones

Stefan K. Grebe, M.D., of the Division of Clinical Biochemistry & Immunology and the Department of Laboratory Medicine and Pathology at Mayo Clinic in Rochester, Minn., says: "Tests for free thyroid hormone are among the most frequently ordered laboratory tests, despite consensus that measurement of thyrotropin (thyroid-stimulating hormone or TSH) should usually suffice."

Dr. Grebe explains that peripheral thyroid hormone testing should be limited to a few clinical scenarios:

• Confirmation of hypothyroidism or hyperthyroidism in cases with borderline abnormal serum TSH concentrations
• Assessment of severity of hyperthyroidism or hypothyroidism
• Rapidly changing thyroid hormone levels, such as during treatment of thyrotoxicosis
• An unreliable pituitary-thyroid feedback loop, such as pituitary disease
• TSH assay interferences due to TSH autoantibodies or heterophilic antibodies

Range of result differences of thyroid function tests

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Dr. Grebe notes: "In many such cases, measurement of total thyroid hormones is just as informative as testing for free thyroid hormones while being analytically more reliable. Moreover, free thyroid hormone assays are only marginally less susceptible to interferences from drugs or nonthyroidal illness than total thyroid hormone assays."

Dr. Grebe continues: "Heparin, for example, and low-molecular-weight heparin elevate lipoprotein lipase levels, creating increased circulating concentrations of free fatty acid. These fatty acids displace thyroid hormones from binding proteins, elevating free thyroxine (FT-4) and free triiodothyronine (FT-3) levels, in some cases more than twofold above the upper limit of the reference ranges."

Ravinder J. Singh, Ph.D., in the Division of Clinical Biochemistry & Immunology and the Department of Laboratory Medicine and Pathology at Mayo Clinic in Minnesota, explains: "Even in the absence of interferences, many FT-4 and FT-3 assays give inaccurate results in some cases. Most of these assays use thyroid hormone analogues, designed to not displace thyroid hormone from binding proteins while competing with the patient's free thyroid hormone for assay antibodies."

Dr. Singh says: "This assay design works only over a relatively narrow range of concentrations of binding proteins and thyroid hormones. Reference methodologies using physical separation of bound thyroid hormone from free thyroid hormone solve this problem, but they are only available for FT-4, have longer turnaround times, and continue to be susceptible to interferences related to drugs or illness."

Measurement of circulating thyroid-stimulating immunoglobulins

Thyrotoxicosis affects about 3 million new patients in the United States each year. More than 60 percent of cases are caused by Graves' disease, a disorder characterized by production of autoantibodies (thyroid-stimulating immunoglobulins [TSIs]) that stimulate the TSH receptor. Since TSIs are disease specific and are detectable in more than 90 percent of patients with Graves' disease, they reliably distinguish Graves' disease from other causes of thyrotoxicosis.

Dr. Singh notes that there are two different types of clinical assays for TSI detection:

• TSH receptor autoantibody-binding (TRAB) assays
• TSI bioassays

"In TRAB assays, labeled TSH competes with TSI in patient serum for binding to assay TSH receptors," says Dr. Singh. "TSI bioassays use cell lines that express the TSH receptor and a cyclic adenosine monophosphate (cAMP)-controlled luciferase gene. When these cells are exposed to TSIs, cAMP is produced and thus drives luciferase production, which in turn leads to light production upon cell lysis and substrate addition."

Serial dilution curves of international standard material of thyroid-stimulating immunoglobulins

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Dr. Grebe explains: "TRAB assays and TSI bioassays show about 90 percent agreement in detecting TSIs, with the latter being somewhat more sensitive at low TSI concentrations and the former possibly giving more accurate results at high TSI concentrations."

"Either assay is more accurate (and cheaper) than a radioactive iodine uptake and scan, which are traditionally used to differentiate Graves' disease from other causes of thyrotoxicosis," says Dr. Grebe. "TRAB assays and TSI bioassays are also particularly useful in distinguishing hyperemesis gravidarum-related thyrotoxicosis from a first-trimester presentation of Graves' disease."

Dr. Grebe continues: "Another key application during pregnancy is risk assessment for fetal-neonatal Graves' disease. This disorder can occur in pregnant women who had previous thyroid-ablative treatment for Graves' disease. These women have normal thyroid function test results, but they might still be producing TSIs, which can pass through the placenta to the infant and cause fetal thyrotoxicosis. Results of maternal TRAB assay or TSI bioassay that are more than two or three times the upper limit of the reference ranges are correlated with fetal thyrotoxicosis, indicating a need for high-risk obstetric care and serial TRAB assays or TSI bioassays."