Endocrine Abstracts

Introduction: There continues to be much discussion around optimization of thyroid hormone status in hypothyroid individuals. Over 10 million thyroid function tests (TFTs) are carried out in England each year, most requests coming from primary care. This study aimed to investigate how thyroid hormone levels change in the years pre- and post-initiation of levothyroxine (LT4).

Methods: Using citywide population health records, we analysed TSH/FT4 results during the period 2012-2023(12 years) for patients diagnosed with primary hypothyroidism and whose medication started within the period 2015-2019(5 years). Only patients with more than total of 5-TFTs during the test period were included. Reference ranges relevant to diagnosis were thyroid stimulating hormone(TSH) maximum 4.2miU/l / free thyroxine(FT4) minimum 12.0pmol/l.

Results: In the years prior to medication starting, TSH increased progressively more rapidly. Once on medication TSH fell to levels at or below TSH level even several years before diagnosis (3.1mu/l vs historical 3.5mu/l). Average daily levothyroxine dose continued to increase over the years post-initiation from 49 mg to 69 mg/day at 7-years with a corresponding rise in median FT4 from 13.0 to 16.1pmol/l. Median FT4 was higher than historical values from the end of year11 of treatment. 60% of women by age 50 years had started replacement treatment vs 50% of men, in keeping with younger age of onset. Levels of TSH were higher in individuals aged 60-years or more before LT4 initiation and remained so afterwards.

Conclusion: This population-based study reflects varying responses in different patients, but with overall further evidence for the increasingly unphysiological’ TFT profile in treated hypothyroid individuals, in the years post-levothyroxine initiation. Both levothyroxine dose and measured FT4 continuing to increase significantly in the years after levothyroxine started, suggesting that dose adjustments may in some cases be driven by patients’ perceived lack of benefit rather than biochemical profile.