Substantial Variability Persists in Thyroid Function Tests

Ordering the appropriate thyroid panel is wrong 30% of the time, and the right tests are missed equally as often, prompting discussion of need for clearer guidelines, and reliance on individualized care, in the meantime.

With Robert Schmidt, MD, PhD, and Scott Isaacs, MD

Researchers surveyed 82 sites nationwide and discovered substantial variation in thyroid testing practices,1 according to findings published in the journal ThyroidTheir analysis focused on tests for thyroid stimulating hormone (TSH), free thyroxine (FT4), total thyroxine (TT4), free triiodothyronine (FT3), total triiodothyronine (TT3), triiodothyronine uptake (T3U), reverse triiodothyronine (rT3), and complete blood counts (CBC).

“Our study found significant variation in the way that physicians order thyroid tests,” said lead author Robert Schmidt, MD, PhD, MBA, associate professor of pathology and director of the Center for Effective Medical Testing at the University of Utah Health Sciences Center in Salt Lake City.

“It is unlikely that the variation could be explained by differences in patient populations, suggesting that there is uncertainty regarding the best way to approach thyroid testing or that some organizations are not ordering thyroid tests correctly.”

The way physicians order thyroid labs is highly variable.

Inconsistencies in Laboratory Panel

The authors noted several other interesting findings:1

  • Inpatients had a much lower thyroid work-up rate (TSH/CBC) compared to outpatients (0.03 for the former; 0.26 for the latter).
  • Nearly all T4 testing was for FT4 rather than TT4, while FT3 and TT3 rates were comparable.
  • Reports of rT3/TSH and T3U/TSH ratios showed most variability.

“I do a lot of research on laboratory test utilization,” Dr. Schmidt told EndocrineWeb. Our findings build on results of a Harvard-led study that indicated about 30% of clinical lab testing is inappropriate and as many necessary tests go unordered, he said.

“We've confirmed that clinicians order the wrong test, unnecessary tests, or tests at the wrong time,” he said, “This wastes money and can lead to suboptimal care,"

In addition, the problem may persist because “test utilization is usually evaluated relative to a guideline. However, there are many more tests than guidelines,” so we are left unable to evaluate lab profiles for which there are no guidelines, said Dr. Schmidt.

How to Gauge Needed Thyroid Labs?

Until guidelines become available, one approach may be to compare test patterns across organizations and look for practice variations,” said Dr. Schmidt, “The hypothesis is that large variations in practice may reflect inappropriate testing.”

Also, benchmarking studies like this may be another way to identify practice variation—an indication of inappropriate ordering—but does not identify best practices, he said.

Given what we know, our recommendation is for "screening with TSH followed by free T4 if TSH is abnormal.  Free T4 is preferred to total T4, and our study shows that most labs provide free T4,” Dr. Schmidt said. Moreover, "a T3 uptake is an outdated test that is still ordered by some physicians. Yet, there is no consensus on whether free T3 or total T3 is better and we observed very significant practice variation with respect to this test.”

Focus More on Individualized Clinical Care

Scott Isaacs, MD, medical director of Atlanta Endocrine Associates, acknowledged that there is significant variability in thyroid function testing as confirmed by the findings of this study. Thyroid hormone testing (T4, T3, free T4, free T3) can have variability depending on the laboratory assay used.

“The TSH is the most consistently reliable test. Differences in laboratory technique and standardizations can result in variability. The rT3 test is especially unreliable and often misleading, whereas TSH tends to be more reliable with less lab-to-lab variability,” Dr. Isaacs told EndocrineWeb.

To lessen the problems presented by variability, “it is always important to remember that you are treating a patient, not a number. Managing thyroid disorders requires clinical expertise in interpreting laboratory values in conjunction with the patient’s symptoms,” he said. 

"We must not rely too heavily on a single laboratory value since variability is seen across all blood tests. Rather, clinical decisions are best made using the patient's history, symptoms, and physical exam findings, along with blood test results—not lab tests alone," said Dr. Isaacs, "That said, most errors in interpretation occur when patients try to interpret their own labs."


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