A publication of the American Thyroid Association

Summaries for the Public from recent articles in Clinical Thyroidology

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A high proportion of serum rT3 tests are ordered by a relatively small number of providers in the United States

CTFP Volume 12 Issue 1

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.

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.

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.

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


support thyroid researchTSH: thyroid stimulating hormone – produced by the pituitary gland that regulates thyroid function; also the best screening test to determine if the thyroid is functioning normally

Thyroxine (T4): the major hormone produced by the thyroid gland. T4 gets converted to the active hormone T3 in various tissues in the body.

Triiodothyronine (T3): the active thyroid hormone, usually produced from thyroxine. Reverse

Triiodothyronine (rT3): the inactive thyroid hormone produced from thyroxine in various tissues in the body. Levels of T3 and rT3 are exactly opposite each other.