You will need to make it vomit them up as soon as possible. Grab your 3% Hydrogen Peroxide and a spoon or syringe. The dose is about 1 teaspoon or 5cc per 10 pounds of body weight. Follow with equal parts of water and then take it outside for a little walk in the yard. Vomiting should happen in 15 to 20 minutes. If not you can repeat the peroxide and water only once. If that doesn't induce vomiting you may have to take it in. 50 tablets is a lot.
Clinical signs of thyroid hormone toxicosis in dogs include hyperactivity, lethargy, tachycardia, tachypnea, dyspnea, abnormal pupillary light reflexes, vomiting, and diarrhea. High overdoses of levothyroxine sodium in dogs should be managed by decontamination (vomiting) and administration of activated charcoal followed by supportive care.
That is probably about 30 times the dose he would take in a day for a regular dog his size. So there is a good chance you can see side effects. It might be best to take him in for activated charcoal (this is not grilling charcoal). It will prevent the medication from being absorbed as well and metabolized by the body.
Yes, it could kill him, probably not likely if he isn't having any signs now. That is why it would have been a good idea to get the activated charcoal in his sysem.
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