Long-term use of levothyroxine is associated with adverse health effects.

Caveats

• The adverse effects documented in the literature are predominantly associated with overtreatment (TSH suppression) or excessive dosing, not with properly monitored replacement therapy — a critical distinction the claim omits.
• Key supporting studies, including the mortality signal in older adults (Source 4) and cancer association (Source 7), are observational and subject to significant confounding and indication bias, meaning sicker patients may have been more likely to receive the drug.
• The most rigorous recent evidence (2026 systematic review; 2025 cardiovascular studies) finds levothyroxine at replacement doses is generally safe and may even reduce cardiovascular mortality and major adverse cardiac events.