How Do Antithyroid Agents Work?

Antithyroid agents are a class of drugs used to treat an overactive thyroid gland caused by hyperthyroidism (when the thyroid gland produces more thyroid hormones than the body needs) and Graves’ disease (an immune system disorder that results in the overproduction of thyroid hormones) and prepare the thyroid gland for surgical removal.

Antithyroid agents work by inhibiting or suppressing the synthesis of thyroid hormones or decreasing thyroid hormone activity. They are also indicated for use in the radioactive iodine uptake test to evaluate thyroid function and protect the thyroid gland in a radiation exposure emergency.

Antithyroid drugs decrease the levels of two hormones produced by the thyroid gland (located at the front of the neck)--thyroxine (T4) and triiodothyronine (T3), which are formed by combining iodine (from food) and a protein called “thyroglobulin” with the assistance of an enzyme called “peroxidase.”

These hormones play an important role in the regulation of weight, energy levels, internal temperature, skin, hair, nail growth, etc. The hypothalamus and the pituitary gland, which are located in the brain, help control the thyroid gland. The hypothalamus releases thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to release thyroid-stimulating hormone (TSH). When the hypothalamus and pituitary are working normally, they sense when:

• thyroid hormone levels are low, so they secrete more TRH and TSH, stimulating the thyroid to make more hormones.
• thyroid hormone levels are too high, so they secrete less TRH and TSH, reducing hormone production by the thyroid.

Hyperthyroidism (overactive thyroid) occurs when the thyroid gland produces too much of the hormone thyroxine and the symptoms include:

• Weight loss
• Fast heart rate
• High irritability/nervousness
• Muscle weakness 
• Tremors (involuntary, rhythmic muscle contraction leading to shaking movements in one or more parts of the body)
• Infrequent menstrual periods
• Sleep problems
• Eye irritations 
• Heat sensitivity

Antithyroid agents work in the following ways:

• They suppress the excessive production of the thyroid hormones (T3 and T4) and decrease their activity.
• In a radiation emergency, they block the thyroid gland from absorbing radioactive iodine, protecting it from damage and thus reducing the risk of thyroid cancer.
• They loosen and break up mucus in the airways, helping in coughing up the mucus which makes breathing easier in long-term lung problems.

Antithyroid agents are used:

• To treat:
  • Hyperthyroidism (when the thyroid gland produces more thyroid hormones than the body needs)
  • Thyroid cancer
  • Thyrotoxic crisis (an acute, life-threatening, hypermetabolic state induced by excessive release of thyroid hormones)
  • Graves’ disease (an immune system disorder that results in the overproduction of thyroid hormones)
• Preoperative thyroidectomy (surgical removal of the entire or a part of the thyroid gland)
• Before radioactive iodine therapy

Some of the common side effects include:

• Diarrhea
• Nausea
• Vomiting
• Weakness
• Abdominal pain
• Headache
• Metallic taste in mouth

Other rare side effects include:

• Rash
• Fever
• Joint pain
• Swelling of the face, lips, tongue, throat, hands, or feet
• Trouble breathing, speaking, or swallowing
• Irregular heartbeat
• Chest pain
• Drowsiness (feeling abnormally sleepy during the day)
• Dizziness (feeling faint, weak, or unsteady)
• Sore throat
• Yellowing of eyes/skin
• Dark urine
• Leucopenia (low white blood cell count)
• Thrombocytopenia (low blood platelet count)
• Vertigo
• Salivary gland swelling
• Aplastic anemia (when the bone marrow does not make enough red and white blood cells and platelets)

Information contained herein is not intended to cover all possible side effects, precautions, warnings, drug interactions, allergic reactions, or adverse effects. Check with your doctor or pharmacist to make sure these drugs do not cause any harm when you take them along with other medicines. Never stop taking your medication and never change your dose or frequency without consulting your doctor.

Generic and brand names of antithyroid agents include:

• Hicon
• Losat
• Methimazole
• Northyx
• Pima Syrup
• Potassium iodide
• Propylthiouracil
• PTU
• Sodium iodide I-131
• SSKI
• Tapazole
• ThyroSafe
• ThyroShield