Levothyroxine

What is this medication for?

• Hypothyroidism (replacement therapy)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Tachycardia, polyphagia, PU/PD, excitability, panting (overdose signs) (Only with overdosing)
• "Apathetic" hyperthyroidism (listlessness, loss of appetite) (~10% of overdosed cats)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Acute MI, thyrotoxicosis, untreated adrenal insufficiency
• has or has had: Treated hypoadrenocorticism, cardiac disease, diabetes, geriatric patients — start low

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Oral antacids, cholestyramine, ferrous sulfate, sucralfate, high-fiber diet
  • Antidiabetic agents
  • Warfarin
  • Phenobarbital, rifampin
  • Tricyclic antidepressants

Storage and handling

Room temperature, protect from light/heat/humidity — inherently unstable, especially in compounded formulations (limit compounded product use to 10–15 days storage).

Good to know

• Brands are not necessarily bioequivalent (no FDA approval for veterinary products) — if switching brands or from tablet to liquid, retest T4 levels rather than assuming an equivalent dose.
• Canine doses are much higher (per kg) than human doses — flag this explicitly if a human pharmacy is filling the prescription, to avoid under-dosing errors.
• Reduce starting dose in older patients or those with cardiac disease/diabetes — start low, titrate up with monitoring.