TSH-Lowering Levothyroxine Therapy Is Associated with Higher rT3 and Stable fT3 Levels in Euthyroid Patients with Papillary Thyroid Microcarcinoma During Active Surveillance.

Furumura Y, Miyauchi A, Ito M, +7 more, Akamizu T.

Journal ArticleAbstract only — full text under license

AI summary — for doctors, generated from the abstract only

In euthyroid patients with papillary thyroid microcarcinoma on active surveillance, mild TSH-lowering levothyroxine was associated with higher reverse T3 and unchanged free T3.

What was studied

A retrospective study of patients with PTMC under active surveillance treated with levothyroxine, grouped by final TSH (209 low-normal, 110 mildly subnormal) and compared with age- and sex-matched untreated controls for TSH, fT4, fT3, T3, and rT3.

Key findings

Both treated groups had significantly higher fT4 than matched controls; specific statistic not fully captured in the abstract
rT3 was higher while fT3 remained stable with levothyroxine therapy
A correlation was reported (r = 0.51); specific statistic not fully captured in the abstract

Clinical relevance

Raises the possibility of a compensatory buffering mechanism in thyroid hormone metabolism during mild TSH suppression; this is mechanistic and does not establish clinical benefit or harm.

Who should care

Endocrinology

Limitations

Retrospective design; key comparative statistics are truncated in the abstract.

Mild TSH suppression with levothyroxine appeared to raise reverse T3 while keeping free T3 stable.

This is an AI-generated summary of the published abstract, not a clinical recommendation. It may not reflect the full text or subsequent updates — verify against the original source before acting on it.

Abstract

Background

We previously reported that lowering serum thyrotropin (TSH) levels with levothyroxine (LT4) during active surveillance (AS) of papillary thyroid microcarcinoma (PTMC) was associated with a trend toward a lower rate of disease progression. In euthyroid patients with PTMC managed with mild TSH-lowering therapy, free thyroxine (fT4) levels increased, whereas free triiodothyronine (fT3) levels remained unchanged overall, thereby raising questions regarding thyroid hormone metabolism. We therefore investigated thyroid hormone balance, with particular focus on thyroxine (T4) metabolism, including serum reverse triiodothyronine (rT3).

Methods

This retrospective study included patients with PTMC undergoing AS who were initially euthyroid and treated with LT4. At the final follow-up, patients were classified according to the detailed TSH score: 209 in the low normal group (lower half of the reference range) and 110 in the high subnormal group (mildly subnormal range). Age- and sex-matched controls were selected from euthyroid PTMC patients undergoing AS who had not received LT4. TSH, fT4, fT3, triiodothyronine (T3), and rT3 levels were analyzed. TSH, fT4, and fT3 were measured using an electrochemiluminescence immunoassay, and T3 and rT3 were measured using liquid chromatography-tandem mass spectrometry.

Results

Compared with matched controls, both the low normal and high subnormal groups showed significantly higher fT4 levels (p p r = 0.51, p < 0.001), whereas no significant correlations were observed with TSH, fT3, or T3.

Conclusions

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. These findings raise the possibility of a compensatory buffering mechanism in thyroid hormone metabolism.

DOI

10.1177/10507256261484710

PMID

42675828

Research metadata (titles, abstracts, authors, publication dates) is sourced from Europe PMC, which aggregates content from PubMed/MEDLINE, PubMed Central, and other providers under the U.S. National Library of Medicine (NLM/NCBI). NLM does not claim copyright on abstracts; the original journal publishers or authors may hold rights to the full articles. AI-generated summaries are derived only from the abstract, never the full text, and are always labeled as AI-generated.

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