Hypothyroidism

• The hormones T3 and T4 are produced by the thyroid, a gland with two sides, one on each side of the dog's windpipe. T4 is an inactive form of T3. Cells can convert T4 into T3. Hypothyroidism will usually cause some sort of skin abnormality, such as loss of fur on parts of the body. It can also cause obesity, lethargy and anemia.
  
  

Focal Seizures

• Seizures can be terrifying for an owner to watch. A focal seizure is limited to one area of the brain. If this is in the part of the brain that affects movement, then it is called a simple partial seizure. This is usually characterized by twitching of facial muscles, often on one side of the face. This can eventually spread to the legs on the same side of the body. A complex partial seizure occurs in the part of the brain that affects mood and behavior. This is characterized by odd behavior that may include hysterical running, resonant vocal sounds, crouching, vomiting, diarrhea, frothing mouth and abnormal thirst.
  
  

Phenobarbital

• Phenobarbital is the most common drug used to help prevent seizures in humans and dogs. This drug can cause abnormally low levels of T4, but this is not considered hypothyroidism, and the dog does not need to be treated for it. However, dogs taking Phenobarbital can have a false positive on traditional tests for hypothyroidism. A test that won't be confused by the use of Phenobarbital is called the equilibrium dialysis free T4 test.
  
  

Causes of Seizures

• There are numerous possible causes for seizures, including brain tumors, high fever, hypoglycemia (low blood sugar) and many different diseases. Seizures with no apparent underlying cause and continue to recur are referred to as epileptic seizures. Hypothyroidism can cause seizures in rare cases, which is why the need to test a dog on Phenobarbital for hypothyroidism does occur.
  
  

Treatment

• Hypothyroidism is treated with pills of the T4 hormone. These are given once or twice a day for the rest of the dog's life. If the hypothyroidism is causing seizures, the seizures should stop when the hypothyroidism is under control. It would be extremely rare, but it is possible, for a dog to have hypothyroidism and focal seizures caused by something else. The dog would then need to take Phenobarbital as well as the T4 pills if the underlying cause for the seizures cannot be determined or cured.