BACKGROUND
Different types of laboratory tests are available for evaluation of thyroid disorders. These tests are used mainly to measure either hormones (like TSH, T₄ and T₃) or thyroid antibodies. Extensive clinical studies have been done to guide clinicians to choose the most effective test in every clinical situation.

Reverse T₃ (rT₃) is not one of the primary hormones produced by thyroid gland. It is mostly produced inside the cells in other tissues from the breakdown of T₄. The normal pathway is that T₄ is broken down to T₃, which is the active hormone. T₄ can also be broken down to rT₃, which is an inactive hormone. The production of T₃ and rT₃ is exactly opposite: the higher the T₃ level, the lower the rT₃ level; the lower the T₃ level, the higher the rT₃ level. Because of this relationship, and the fact that rT₃ is inactive, most clinicians do not use measurement of rT₃ for assessing a patient for hypothyroidism or hyperthyroidism. Still, some providers continue to order rT₃ in the assessment of thyroid function. This study was done to identify the ordering pattern of serum rT₃ by clinicians.

THE FULL ARTICLE TITLE:
Schmidt RL et al. Does reverse triiodothyronine testing have clinical utility? An analysis of practice variation based on order data from a national reference laboratory. Thyroid 2018 Jul; 28(7): 842-848.

SUMMARY OF THE STUDY
The data from the National Reference Laboratory was used to conduct this study. The authors reviewed all the thyroid related tests that had been ordered from November 2015 to November 2016. They studied the records to find out the ordering pattern of the tests based on hospital types (for example, community versus academic hospitals) and providers (type of specialty). They also reviewed the published medical articles about rT₃ and did a Google search to identify the information available in internet to public about rT₃.

They found that relatively small proportion of providers had ordered majority of rT₃ tests. Of the 100 providers who had ordered rT₃ the most, 60% were practitioner of functional medicine and 40% conventional medicine (thyroid specialists, internal medicine, family medicine and gynecologist). Functional medicine was defined as a form of alternative medicine often based on complementary and holistic treatment techniques.

The review of medical articles about this topic showed that 90% were published before 2000 and provided limited evidence to support measuring rT₃ for evaluation of regular thyroid disorders like hypothyroidism and hyperthyroidism. Most of the publications were about the effect of medications, medical conditions other than thyroid disease and severe illness on rT₃ level.

The Google search showed mostly links to published articles in medical journals. Most of the webpages that discussed the use of rT₃ level for clinical purposes were 8-fold more associated with functional medicine than conventional medicine.

WHAT ARE THE IMPLICATIONS OF THIS STUDY?
This study suggests that the majority of rT₃ tests are ordered by a relatively small proportion of clinicians, mostly providers practicing functional medicine. The review of medical literature does not support routine measurement of rT₃ in clinical care of a patient with hypothyroidism and hyperthyroidism. This is important for patients who have thyroid disease and would like to be involved in decision making regarding their care. This might be especially important for patients who have to pay or share the cost of their laboratory tests.

— Shirin Haddady, MD

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