Levothyroxine
1,379 adverse event reports submitted to the FDA
Active Ingredients
Administration Routes
Species Affected
Most Affected Breeds
Most Reported Reactions
Outcome Breakdown
Data Summary
| Metric | Value |
|---|---|
| Total adverse event reports | 1,379 |
| Reports involving death | 159 |
| Case-fatality rate (reported events) | 1150.0% |
| Distinct species in reports | 5 |
| Distinct breeds in reports | 20 |
| Distinct reactions reported | 20 |
| Active ingredients on file | 1 |
Source: FDA Center for Veterinary Medicine — Adverse Event Reporting (CVM AER). Counts reflect voluntary reports only.
Levothyroxine Adverse Event Insights
The FDA Center for Veterinary Medicine database currently holds 1,379 adverse event reports referencing Levothyroxine, including 159 reports in which the animal died — a 1150.0% case-fatality figure among reported events only, not a population-level mortality rate. Active ingredient on file: Levothyroxine. Reported administration routes include Oral, Unknown, Ophthalmic. These numbers reflect voluntary submissions from pet owners, veterinarians, and manufacturers and therefore under-represent mild events and over-represent severe ones — a pattern the FDA has documented repeatedly for pharmacovigilance datasets.
The species most frequently named in Levothyroxine reports are Dog (1,312 reports), Human (44 reports), Cat (16 reports), with Dog accounting for the largest share. Within those species, Retriever - Labrador (204), Retriever - Golden (129), Crossbred Canine/dog (76) appear most often — though breed popularity and ownership density shape these counts as much as any drug-specific sensitivity. This distribution matters because the same active ingredient can behave very differently across body sizes, ages, and species physiology.
The most commonly reported clinical signs associated with Levothyroxine are Vomiting (218), Diarrhoea (158), Lethargy (see also Central nervous system depression in 'Neurological') (143), Elevated alanine aminotransferase (ALT) (103). Of the 1,362 reports with a coded outcome, Ongoing is the leading category at 35.8%. Because FDA adverse event data describes correlation rather than causation, these figures are best used to frame informed questions with a veterinarian and to compare reporting patterns across related products — not as a standalone safety verdict on Levothyroxine.
Source: FDA Center for Veterinary Medicine — Adverse Event Reports FDA Center for Veterinary Medicine — Adverse Event Reports Data reflects voluntary submissions and may not represent actual incidence rates
Read our methodology — how this data is sourced, computed, and verified.