Clin Endocrinol

Most levothyroxine‑treated patients fall outside standard TSH ranges

February 3, 2026

In a 14‑year analysis of 47,869 individuals with diagnosed hypothyroidism and 393,101 untreated individuals, UK researchers assessed simultaneous TSH and FT4 results by levothyroxine dose. FT4 distributions in treated patients were similar in shape to those of untreated individuals but were shifted higher, with greater separation at higher doses. TSH distributions differed substantially: treated patients showed a marked shift toward low or undetectable levels, with increasing skewness as doses rose. Among untreated individuals, 90.3% were within the TSH reference range, compared with 43.8% of those on levothyroxine. Median doses were higher in men across age groups. Treated and untreated populations showed similar TSH values only around FT4 ≈20 pmol/L.

Clinical takeaway: Consider that levothyroxine therapy may produce TSH and FT4 patterns that differ from those seen in untreated euthyroid individuals when interpreting thyroid function tests.

Source:

Heald AH, et al. (2025, December 22). Clin Endocrinol (Oxf). Evaluating the Link Between Thyroid Function Test Results and Levothyroxine Dose in the Management of Hypothyroidism: Can We Improve Dosing Regimes? https://pubmed.ncbi.nlm.nih.gov/41429661/