Evaluating the Link Between Thyroid Function Test Results and Levothyroxine Dose in the Management of Hypothyroidism: Can We Improve Dosing Regimes?
Authors' conclusion
We have here described that distribution of FT4/TSH is different in people on and off Levothyroxine treatment. For those on Levothyroxine, only 43.8% were within the TSH reference range and the degree of difference increased in treated individuals with Levothyroxine daily dose. The potential implication of our findings is that clinicians must be mindful as they diagnose and treat hypothyroidism that the administration of Levothyroxine, while in most but not all individuals is clinically beneficial does not return the individual to the same balance of TSH and FT4 as seen in euthyroid individuals.
Study context
- Population
- Diagnosed hypothyroid individuals on LT4 and untreated/euthyroid individuals with simultaneous TSH/FT4 results, city-wide population record (England)
- Participants
- 47,869 vs 393,101
- Exposure
- LT4-treated vs untreated/euthyroid individuals
- Levothyroxine-treatedn = 47,869
- Untreated/euthyroidn = 393,101reference
- Outcome
- Distribution of TSH and FT4 by LT4 dose vs untreated individuals
Methodological detailscity-wide population record, 14 years
- City-wide population record
- 14 years
Main finding
As reported in the abstract
| Outcome | Levothyroxine-treated |
|---|---|
| Within TSH reference range | 43.8% |
- FT4 distribution similar in shape but shifted to higher FT4 with LT4, even at lowest dose; separation increased with dose (F value)
- 1.5 increasing to 4.2
- TSH similar in treated and untreated only at FT4 ≈ 20 pmol/L; below, treated have higher TSH, above, lower TSH for same FT4
- 20pmol/L
Observational data: associations, not effects.
Further findings
Median LT4 daily dose by sex, age 50-59 years (highest)
Median LT4 daily dose by sex, age 50-59 years (highest)
- Median LT4 dose higher in men than women in all decades
- In treated individuals, median T4 rose and TSH fell progressively with age (women > men)
Original publication
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