Drug profile

The FDA adverse-event record for L-Thyroxine

32 adverse-event reports submitted to the FDA Center for Veterinary Medicine reference L-Thyroxine. Species, reactions, and outcomes follow.

#1401 of 2,679
by report volume (all drugs)
32
adverse-event reports
21.9%
death-coded share
Dog
most-reported species
Verify with FDA CVM →

Corpus placement

According to the FDA Center for Veterinary Medicine openFDA adverse-event extract, L-Thyroxine ranks #1387 of 2679 by total reports (32). Death-coded-share rank requires at least 50 reports. National dual: Milbemycin Oxime + Spinosad is #1 of 2679 drugs by total adverse-event reports (149,624) at death-coded-share rank 965 of 1130 drugs with at least 50 reports (2.3%); separately, Atlantic Salmon Transgenic Whole (Opafp-Ghc2 Rdna Construct Inserted Into The A-Locus In The Eo-1A Lineage) is #1 highest death-coded share at 97.8% (report rank #821).

149,624
Milbemycin Oxime + Spinosad reports (#1)
97.8%
Atlantic Salmon Transgenic Whole (Opafp-Ghc2 Rdna Construct Inserted Into The A-Locus In The Eo-1A Lineage) death share (#1)
#1387
This drug, reports of 2679

Report volume reflects how common a breed or drug is and how often owners and vets file — not actual risk. Death-coded share ranks are limited to entries with enough reports to be meaningful. The two maps do not agree at #1.

Release-over-release change

What changed in the latest FDA data snapshot

PlainBreed reconstructed the prior snapshot from dated openFDA partitions through 2025 Q4, then compared it with the 2026-07-27 openFDA export. L-Thyroxine has 32 reports now versus 32 in that prior snapshot: 0 the same number of (0.0%).

Report change
+0
Death-coded reports
+0
Death-coded share
+0.0 pts

This compares reported-event counts in two source snapshots. It does not establish a change in real-world risk, product safety, or disease incidence.

Subscribe to L-Thyroxine changes

Signature read: L-Thyroxine's 21.9% death-coded share is among the highest in the openFDA drug corpus.

Important: Adverse event reports do not establish that a drug caused or contributed to the event. Consult your veterinarian before making treatment decisions.
32
Total Reports
7
Deaths Reported
21.9%
Death-coded share of reports

Active Ingredients

L-Thyroxine

Administration Routes

OralUnknown

Species Affected

Dog 32

Most Affected Breeds

Retriever - Labrador 5
Boxer (German Boxer) 3
Shar Pei 3
Beagle 2
Chihuahua 2
Retriever - Golden 2
Collie - Border 1
Spaniel - Cocker American 1
Spaniel - Springer English 1
Sheepdog - Shetland 1

Most Reported Reactions

Anorexia 6
Other abnormal test result NOS 5
Vomiting 5
Lethargy (see also Central nervous system depression in 'Neurological') 5
Diarrhoea 4
Lack of efficacy - NOS 4
Weight loss 4
Death 4
Seizure NOS 3
Elevated serum alkaline phosphatase (SAP) 3
Death by euthanasia 3
Hypersalivation 3

Outcome Breakdown

Ongoing
10 (31.3%)
Outcome Unknown
8 (25.0%)
Recovered/Normal
7 (21.9%)
Died
4 (12.5%)
Euthanized
3 (9.4%)

L-Thyroxine: high death-coded share in the FDA AER row

Read these counts as reporting volume, not risk: per our methodology, a widely-prescribed drug will rack up more raw FDA reports in a given year than a rarely-used one at an identical true rate, so we recommend comparing within similar-exposure products rather than across the full 2025 catalog.

What to do with this drug

A report does not prove a drug caused the reaction; consult a veterinarian before acting on this data.

Source: FDA Center for Veterinary Medicine openFDA Animal & Veterinary Adverse Events (CVM AER) · 2025

openFDA Animal & Veterinary Adverse Events (FDA CVM) for L-Thyroxine (#1401 of 2,679 by report volume); death-coded 21.9%. PlainBreed drug row · death-coded share stamp 21.9% · vintage 2025.

Informational only. FDA adverse-event reports are voluntary and do not prove causation; consult a veterinarian before acting on them. Verify with FDA CVM →