What does the Reverse Triiodothyronine test measure?

Why is Reverse Triiodothyronine measured?

rT3 is elevated in non-thyroidal illness syndrome (euthyroid sick syndrome) — where systemic illness, surgery, caloric restriction, chronic stress, and inflammatory states shift deiodination from the T3-producing pathway to the rT3-producing pathway as an energy conservation response. It is also elevated in cortisol excess states, selenium deficiency, heavy metal toxicity, and certain medications. In functional and integrative medicine, elevated rT3 with low-normal Free T3 — despite normal TSH and Free T4 — is interpreted as evidence of impaired T4 activation, with rT3 occupying T3 receptor sites and producing functional thyroid insufficiency despite apparently normal standard thyroid markers. The clinical validity and treatment implications of the rT3:Free T3 ratio as a decision-making tool for T3 supplementation remain debated in mainstream endocrinology. # REVIEWER FLAG: The clinical utility of rT3 measurement as a basis for T3 therapy decisions is contested in mainstream endocrinology and is primarily a functional medicine clinical application — reviewer to confirm appropriate interpretive framing for the platform's practitioner audience.

Reference ranges

Standard reference

9.2–24.1 ng/dL (14.2–37.2 pmol/L) # REVIEWER FLAG: Confirm Access Medical Laboratories reference range for Reverse T3 — values are assay-specific and rT3 reference intervals vary substantially between laboratories. Confirm assay methodology (RIA vs LC-MS/MS) as methodology significantly affects the numerical range. Also confirm whether rT3 is included in the Access Medical Laboratories Thyroid Panel Comprehensive or is a separate add-on order.

Functional medicine optimal

Free T3:rT3 ratio >20 (ng/dL:ng/dL × 10, or pmol/L:pmol/L as a unitless ratio depending on unit convention) is widely cited as a functional medicine threshold for adequate T3 availability relative to receptor blockade. # REVIEWER FLAG: The Free T3:rT3 ratio threshold and clinical decision criteria are not standardised in mainstream endocrinology and vary between functional medicine practitioners — reviewer to confirm whether a ratio threshold should be cited and which unit convention Access Medical Laboratories reports support.

Units

ng/dL, nmol/L, pmol/L

Reference ranges vary by laboratory and assay platform. Interpret in context of the issuing laboratory's stated range.