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The following is a summary of “How Does Thyroid Hormone Profile Differ on and Off Replacement Treatment?” published in the December 2024 issue of Endocrinology by Heald et al.

Thyroid function tests (TFTs), including free thyroxine (FT4) and thyroid-stimulating hormone (TSH), help assess thyroid health. Optimizing thyroid hormone levels for individuals with hypothyroidism on levothyroxine treatment remains challenging.  

Researchers conducted a retrospective study to compare TFT profiles in individuals receiving levothyroxine treatment and those who were not treated.  

They analyzed TFT results (TSH/FT4) from the Salford Royal Hospital Laboratory (2009–2012) for 12,006 individuals receiving levothyroxine and 43,394 not receiving levothyroxine. A single test result was used for individuals not receiving levothyroxine, and for those on levothyroxine, the median result was considered. Cluster analysis was used to determine the percentage of results outside a predefined boundary (5%-95% percentile values).  

The results showed that individuals not receiving levothyroxine had median TSH and FT4 values of 1.8 mU/L and 15.5 pmol/L, respectively, with 24% of results falling outside the 5%-95% boundary. For those receiving levothyroxine, the median TSH and FT4 values were 2.3 mU/L (+30% vs. untreated) and 18.9 pmol/L (+22% vs. untreated), with 22% of results falling outside the treated boundary. However, 68% of results for those receiving levothyroxine were outside the expected range for untreated individuals (63% of males, 70% of females, 67% of individuals aged under 60, and 64% of individuals aged 60 or older).  

They concluded that levothyroxine treatment often results in TSH and FT4 levels outside the expected range for untreated individuals, particularly for women.  

Source: onlinelibrary.wiley.com/doi/full/10.1111/cen.15185