TSH-Lowering Levothyroxine Therapy Is Associated with Higher Reverse Triiodothyronine (rT3) and Stable Free Triiodothyronine (fT3) Levels in Euthyroid Patients with Papillary Thyroid Microcarcinoma During Active Surveillance
Authors' conclusion
In euthyroid patients with PTMC undergoing AS, LT4 therapy aimed at maintaining TSH within the lower half of the reference range or within the mildly subnormal range was associated with higher rT3 and stable fT3 levels.
Study context
- Population
- Initially euthyroid patients with papillary thyroid microcarcinoma (PTMC) undergoing active surveillance, treated with LT4
- Exposure
- TSH-lowering LT4 therapy vs matched controls without LT4
- Low normaln = 209
- High subnormaln = 110
- Outcome
- Thyroid hormone balance (fT4, fT3, fT3/fT4, rT3, rT3/T3) vs matched controls
- Follow-up
- Final
Methodological detailsage- and sex-matched controls, TSH, free thyroxine (fT4) and fT3 measured by electrochemiluminescence immunoassay; T3 and rT3 by liquid chromatography-tandem mass spectrometry
- Age- and sex-matched controls
- TSH, fT4 and fT3 measured by electrochemiluminescence immunoassay; T3 and rT3 by liquid chromatography-tandem mass spectrometry
Main finding
As reported in the abstract
- fT4 significantly higher in both low normal and high subnormal groups vs matched controls
- p < 0.001
- fT3/fT4 ratio significantly lower, rT3 and rT3/T3 ratio significantly higher in both low normal and high subnormal groups
- all p < 0.001
Observational data: associations, not effects.
Further findings
Correlations of rT3 (low normal and high subnormal combined)
- rT3 positively correlated with fT4
- r = 0.51, p < 0.001
- No significant correlations of rT3 with TSH, fT3 or T3
Original publication
TSH-Lowering Levothyroxine Therapy Is Associated with Higher rT3 and Stable fT3 Levels in Euthyroid Patients with Papillary Thyroid Microcarcinoma During Active Surveillance. Thyroid. Published 31 August 2026. doi:10.
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