LEVOTHYROXINE

(for veterinary information only)

BRAND NAME: THYRO-TAB

BACKGROUND

Thyroid hormone represents the volume dial on one's metabolism. Too much and calories are burned too fast with subsequent heart disease not far away. Too little and the body loses energy, cannot function properly and symptoms arise in multiple organ systems. Hypothyroidism is a classically described hormone imbalance, both in people and in dogs. Details can be found by clicking here.

Fortunately, hypothyroidism can be easily treated by orally providing the hormone that one's diseased thyroid gland is not adequately producing. Symptoms may take several months to resolve depending on the organ systems affected but generally, there is an improvement in energy and attitude after just a few weeks of treatment. The thyroid gland produces thyroxine which is inactive form of thyroid hormone. The body's tissues must activate thyroxine. Levothyroxine is an oral version of the hormone that the thyroid gland produces naturally.

There is a syndrome called "Euthyroid sick syndrome" whereby thyroid levels drop during times of illness as part of the body's reaction to illness. Metabolic processes slow down so the body can concentrate on healing. This is not hypothyroidism and requires no treatment.

Cats rarely become hypothyroid after treatment for hyperthyroidism. Should this occur, treatment with levothyroxine is needed to replace the hormone that is no longer being produced in adequate amounts.

HOW THIS MEDICATION IS USED

Medication is typically given orally once or twice daily. The medication absorbs into the body best on an empty stomach. Because food alters absorption of levothyroxine, it is best to give the medication at a similar time or times daily and on the same schedule with regard to food. Treatment is lifelong.

If a dose is accidentally skipped, do not double up on the next dose. Simply give the medication when it is next scheduled.

Levothyroxine has one use and that is the treatment of hypothyroidism.

SIDE EFFECTS

This medication is associated with very few side effects. Most issues come from giving too high a dose and creating symptoms of hyperthyroidism: excessive thirst and appetite, restless behavior.

Occasionally patients on levothyroxine will experience scaly skin, ear inflammation, lethargy or appetite loss. Any symptoms such as these should be reported to the veterinarian but they are very uncommon and may well have another cause when seen.

INTERACTIONS WITH OTHER DRUGS

There are many medications which will interfere with thyroid tests.

Antacids may interfere with absorption of levothyroxine. A four hour separation between these medications is recommended. A similar situation exists with sucralfate.

Corticosteroid hormones (such as prednisolone) interfere with the activation of thyroxine into active thyroid hormone.

CONCERNS AND CAUTIONS

Patients with concurrent heart disease may need a dose reduction.

Expect monitoring tests to be needed when hypothyroidism is treated.

Page posted: 9/16/2025