Thionamides are a class of oral medications used to manage specific thyroid conditions. Their primary function is to regulate an overactive thyroid gland by controlling hormone production. This makes them a non-invasive treatment option.
Medical Uses of Thionamides
Thionamides are prescribed to treat hyperthyroidism, where the thyroid gland produces excess hormones and accelerates the body’s metabolism. The most common cause is Graves’ disease, an autoimmune disorder where the immune system attacks the thyroid, causing overproduction. Thionamides are a frontline treatment for Graves’ disease, particularly in younger individuals.
These medications are also used in other clinical scenarios. For patients scheduled for a thyroidectomy (thyroid removal surgery), thionamides normalize hormone levels before the procedure. This preparation reduces the risks of surgery on an overactive thyroid. They are also administered before radioactive iodine (RAI) therapy.
The goal of therapy is to restore a normal thyroid state, known as euthyroidism. Treatment duration is often 12 to 18 months to achieve long-term remission. For some patients, prolonged therapy is an effective alternative to more invasive treatments. The length of treatment depends on the patient’s individual response.
How Thionamides Work
The primary action of thionamides is to block the synthesis of new thyroid hormones. They achieve this by inhibiting an enzyme in the thyroid gland called thyroid peroxidase (TPO). TPO is necessary for the oxidation of iodide and its subsequent iodination of tyrosine residues on a protein called thyroglobulin. This disruption halts the creation of thyroxine (T4) and triiodothyronine (T3).
This mechanism interferes with the organification of iodine, preventing it from being incorporated into thyroid hormones. This action directly addresses the cause of hyperthyroidism by reducing the gland’s ability to overproduce hormones.
Thionamides only prevent new hormone synthesis; they do not affect the large stores of pre-formed hormones in the thyroid gland. Because of this, it can take several weeks or months for patients to notice symptom improvement. The stored hormones must be released before the medication’s effects become apparent.
Types of Thionamide Medications
There are two main thionamide drugs: methimazole (MMI) and propylthiouracil (PTU). Methimazole is generally the first-line treatment for most patients with hyperthyroidism. This is due to its longer duration of action, which allows for once-daily dosing, and a lower risk of severe liver injury compared to PTU. Carbimazole, available in Europe and Asia, is converted into methimazole in the body.
Propylthiouracil has more specific indications. It is the preferred drug during the first trimester of pregnancy because of a lower risk of certain birth defects associated with methimazole. After the first trimester, patients are often switched to methimazole. PTU is also favored for treating thyroid storm, a life-threatening complication of hyperthyroidism.
Beyond inhibiting hormone synthesis, PTU also blocks the conversion of T4 hormone into the more active T3 form in peripheral tissues. This dual action provides a more rapid reduction in active thyroid hormone levels, which is advantageous in emergencies like a thyroid storm. Its application is restricted to these specific circumstances due to its less favorable safety profile.
Potential Side Effects and Monitoring
The most frequent adverse effects include skin rashes, itching (pruritus), and hives (urticaria). Some patients may also experience joint pain (arthralgia). These minor side effects can often be managed without discontinuing the medication.
A more serious, though rare, side effect is agranulocytosis, a condition characterized by a severe drop in white blood cells, specifically neutrophils. This leaves the body vulnerable to infections. Patients are educated to immediately report symptoms such as fever or a sore throat to their healthcare provider.
Another significant risk, particularly associated with PTU, is liver injury. Though uncommon, this can range from mild abnormalities in liver function tests to severe hepatitis. Due to this risk, PTU is not recommended as a first-line agent except in specific situations. Methimazole-induced liver injury can also occur but is generally less severe.
Regular medical monitoring is a standard part of thionamide treatment. This involves periodic blood tests to check thyroid hormone levels (T4 and T3) and thyroid-stimulating hormone (TSH) to ensure the dose is effective. Blood tests also monitor for adverse effects, including a complete blood count and liver function tests.