A Phase 2 Study of XP-8121: Once-Weekly Subcutaneous Levothyroxine Sodium for the Treatment of Hypothyroidism in Adults
Authors' conclusion
Once-weekly XP-8121 SC was generally well-tolerated in all treated participants and supported a dose conversion factor of four times the daily oral LT4 dose.
Study context
- Population
- Adults with hypothyroidism on a consistent oral levothyroxine (LT4) dose for ≥3 months, with normal TSH ≥3 months before screening and normal free thyroxine (fT4) at screening39 completed
- Participants
- 46 adults
- Comparison
- Once-weekly SC XP-8121 after switch from daily oral LT4
- XP-8121 SCn = 46
- Outcome
- Final dose conversion factor from oral LT4 to XP-8121 SC (point estimate)
Methodological detailsmulticenter, nonrandomized, open-label, self-controlled
- Multicenter
- Nonrandomized
- Open-label
- Self-controlled
Main finding
As reported in the abstract
Final dose conversion factor point estimate, lower end of range
4.02
90% CI 3.79–4.27
Final dose conversion factor point estimate, upper end of range
4.24
90% CI 4.06–4.42
Further findings
TSH and fT4 normalization among completers on a consistent dose (unchanged for 6 weeks) · end of maintenance
- TSH normalization
- 79.5%
- fT4 normalization
- 100%
- Participants initially underdosed during titration
Patient-reported satisfaction and preference
- Most completers reported higher satisfaction, convenience and perceived effectiveness with once-weekly XP-8121 SC and preferred it over daily oral LT4
Safety
| Outcome | Values |
|---|---|
| TEAEs in participants receiving ≥1 dose | XP-8121 SC 65.2% |
- Treatment-emergent adverse events (TEAEs) in participants receiving ≥1 dose — Total TEAEs reported
- 78
- TEAEs in participants receiving ≥1 dose — Fatigue (most common TEAE)
- 21.7%
- TEAEs in participants receiving ≥1 dose — Injection-site pain
- 10.9%
Original publication
A Phase 2 Study of XP-8121: Once-Weekly Subcutaneous Levothyroxine Sodium for the Treatment of Hypothyroidism in Adults. Thyroid. Published 3 June 2026. doi:10.
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