What medications are most common to treat Hypothyroidism?
Dogs are usually the patients we are dealing with when we talk about hypothyroidism. This is an under active thyroid gland which leads to the signs of the disease.

Levothyroxine, used to treat hypothyroidism after diagnosing by low total T4. Blood tests should be taken 4-6 hours after administration.
At proper levels, there should be no side effects.
Levothyrozine may alter insulin and digoxin requirements. Many drugs may affect t4 levels. Inform your doctor if your pet is receiving any medication when blood levels are obtained.

What can be used to treat Hyperthyroidism?
Cats are usually the patients we see hyperthyroidism in. This is a syndrome where the thyroid is over active. It is usually due to a functional malignancy of the thyroid gland. The gland puts out too much hormone which leads to the disease

Methimazole (Tapazole) used to treat hyperthyroidism. Because of the potential of serious complications, blood tests are very important when using this drug. A CBC is taken before starting Tapazole and total T4, CBC, Liver and Kidney function blood tests will then be performed every 3-6 months to ensure safe and proper dosing.
Patients with liver disease or clotting disorders should not receive the drug. 15-20% of all cats treated develop adverse effects. Notify the hospital if you observe decreased appetite, vomiting, lethargy or any abnormality that concerns you.

Surgery can be used to remove the over active thyroid gland. If both thyroids are removed, then a supplementation of thyroid hormone will need to be started. If only one side is removed, very commonly no medications are needed post operatively.

Radioactive iodine is also used to treat hyperthyroidism. A specialist is needed to administer the product and a week long hospitalization is required. This is a very effective form of treatment but the cost is a limiting factor many times.