Thyroid Poisoning in Dogs

Thyroid hormones are a vital part of the day to day function of the body. Heart rate, body temperature regulation, and oxygen consumption are just a few of the systems that work in sync with the thyroid. Thyroid problems in animals and people are not uncommon making this type of medication a normal item in the medicine cabinets of many households. Whether your pet gains access to your supply of medication or if you accidentally give him to much of his own prescription, veterinary care will be needed to ensure your pet remains in good health. Studies show that dogs can function quite well and have a high margin of safety when it comes to overdose. However, toxicity from the consumption of a large amount is possible, though rarely documented in veterinary literature. A few of the signs of toxicity can be tremors, vomiting, and diarrhea. Treatment is necessary in the case of severe poisoning, and may include medication that must be administered in a hospital setting in order to control signs.

Dogs and humans both can be prescribed thyroid hormones for conditions such as hypothyroidism. The hormone levothyroxine (a synthetic form) is the typical choice for treatment in canines and humans alike. Overdose of this hormone can cause toxicity in your pet.

Signs of thyroid poisoning in dogs may appear between 1 and 9 hours after the pills were ingested. A mild case of toxicity will involve hyperactivity and an increase in heart rate. The ingestion of a large number of thyroid pills may mean the following signs can appear.

• Excitability
• Nervousness
• Excessive panting
• Lethargy
• Vomiting
• Diarrhea
• Abnormal reflex of pupils to light
• Contraction of the pupils
• Tremors of muscles
• Rapid heart rate
• Rapid breathing
• Difficulty breathing
• Aggression

Types

Toxicity to thyroid hormones is typically an acute event, meaning it occurs as the result of a large ingestion of pills. Chronic overdose can occur over time; though studies show that the liver and kidneys can handle and disperse the hormone well even in cases of overabundance. There is also evidence that thyroid hormones have a slow gastrointestinal absorption allowing for a period of toxicity that is not apparent right away.

• Dogs can tolerate and in fact, need a higher dose per weight than humans do in order for the hormone to do its job
• An enzyme in the liver (alanine transaminase) has increased activity; this is why underlying liver issues can exacerbate the poisoning
• Signs will vary in dogs but are most commonly mild to moderate; severe toxicity is rare
• Medications may react adversely to an excess of the hormone
• Concurrent heart problems can make thyroid hormone toxicity more severe

As with any poisoning by medication or substance that is known to be dangerous when ingested, bring along the container or packaging that contained the thyroid hormone pills as this will be helpful for the veterinarian to see. If you have an idea of the time of ingestion this is important information to relay to the veterinarian because whether or not the team induces vomiting depends on the timing (less than 2 hours prior).

Clinical signs relevant to thyroid poisoning will be considered as the physical examination takes place. Your dog’s heart rate and blood pressure will indicate how severe the toxicity is and blood work will reveal T4 levels and other markers like electrolytes and liver enzymes. The treatment protocol will be determined by the level of poisoning indicated in the diagnostic process.

A pet who has experienced significant toxicity due to thyroid hormones will be hospitalized. Fluid therapy will be commenced to aid in the administration of  medications for nausea, vomiting, or diarrhea if needed. Additional drugs could include those meant to regulate signs like a rapid heart rate and uncontrollable tremors, and those which will help to eliminate the excess thyroid hormone from the system. 

If veterinary care is sought within the two hour time frame, inducing vomiting may be initiated along with other gastric decontamination measures such as activated charcoal administration. Oxygen therapy will be provided if beneficial; at the same time the fluid therapy may be slowly continued and the heart activity will be closely watched.

A dog without underlying conditions and who has been treated in a timely manner will have a good prognosis for recovery. Once your pet has been released from the care of the veterinarian, he can return home provided that he has a comfortable, quiet resting place. Continue to monitor him at home and be certain to contact the clinic if your dog’s behavior changes in any way that causes you concern. For the future, keep all medications, dangerous substances, household products, and forbidden food out of reach of your pet and if you have in your possession medication prescribed for your dog, follow the instructions very carefully.