An Evaluation of Longitudinal Thyroid Hormone Levels Over the Years Before and After Patients Started on...

This retrospective study aimed to investigate how free thyroxine (FT4) and thyroid-stimulating hormone (TSH) levels change in the years pre- and post-initiation of levothyroxine (LT4). A better understanding of the relation between thyroid hormone replacement therapy and actual levels of TSH/FT4 in patients could allow more tailored and effective therapy for people with hypothyroidism.

Using de-identified citywide population health records, we were able to analyse TSH and FT4 results during the period 2012-2023 (12 years) for patients whose LT4 was started during 2015-2019 (5 years) and who also had a diagnosis recorded of primary hypothyroidism.

Before LT4 initiation, there was a slight rise in median TSH results over the previous years with a sharper increase in the 12 months prior to medication starting. Once on medication, TSH fell to levels significantly below the pre-medication levels. Between -6 and -2 years before initiation of LT4, median TSH increased from 4.0 to 4.9 mu/L and then in the last year before initiation to 6.4 mu/L. Once on medication the TSH fell to 3.8 mU/L then to 2.7 mU/L over subsequent years, which was significantly below the TSH level before LT4 prescribing was initiated. Median FT4 was low and falling slowly prior to medication starting. Once medication started, FT4 was higher year on year, in relation to increasing doses of LT4 with TSH correspondingly lower. Average daily LT4 dose continued to increase over the years following initiation, with a corresponding rise in FT4. The median LT4 dose for the treated group rose steadily from 49mcg to 69mcg daily.

This population-based retrospective study reflects varying responses in different patients, but with overall further evidence for increasing difference over time in FT4 and TSH in treated hypothyroid individuals versus euthyroid individuals. This has implications for how LT4 dose titration is conducted in treated hypothyroidism.