Antithyroid drugs can reduce the production of thyroid hormones T3 and T4 in case of hyperthyroidism. But what are these drugs and when are they prescribed? How do they work and what are their dangers? Explanations.
Definition: what is an antithyroid drug?
An antithyroid drug is a substance that aims to reduce the production of hormones secreted by the thyroid (T3 and T4). These hormones regulate metabolism and are essential for many body functions. Antithyroid drugs help stabilize thyroid hormone production within weeks (1 to 4 months in general). They can be supplemented by a maintenance treatment. They can also precede treatment with radioactive iodine or thyroid surgery (or removal).
Indications: when to take an antithyroid drug?
Antithyroid drugs are indicated in the treatment ofhyperthyroidismin addition to treatment to relieve the symptoms of hyperthyroidism, radioactive iodine or a thyroid removal. They are for some the first-line drugs in the Graves disease, autoimmune thyroid disease. Antithyroid drugs are also indicated during a overactive thyroid due to nodulesgoiter or iodine overload.
How does an antithyroid drug work?
Antithyroid drugs block the synthesis of thyroid hormones by preventing the action of an enzyme, thyroperoxidase.
Thiouracils such as PTU (propylthiouracil) and high dose BTU also inhibit the enzyme deiodase type 1 located in the liver, kidneys and thyroid, which decreases the transformation of T3 in T4 in the tissues. They prevent the penetration of iodine into the thyroid cells (thyrocytes) and lead to hypersecretion of TSHa hormone secreted by the pituitary gland and which stimulates the thyroid to secrete thyroid hormones. The prescribed dose of antithyroid drugs is dependent on the results of blood tests of T3 and T4it is therefore strictly individual.
Trade name: what are the main antithyroid drugs?
In France, 2 classes of antithyroid drugs are available: sulfur-containing imidazole derivatives such as thiamazole (Thyrozol®) and carbimazole (Neomercazole®) and thiouracils such as propylthiouracil (Propylex®) and benzylthiouracil (Basdene®).
What are the side effects of antithyroid drugs?
Antithyroid drugs may have the following side effects:
- itches,
- skin rashes,
- joint and muscle pain,
- abdominal pain,
- nausea,
- vomitings,
- taste changes,
- fever,
- liver ailments,
- kidney and lung damage,
- a drop in white blood cells (neutropenia or agranulocytosis). This effect is rare and reversible when treatment is stopped, but can be dangerous because the risk of catching infectious diseases is high.
What are the contraindications of antithyroid drugs?
Antithyroid drugs are contraindicated in case of allergy to any of the components of the drug. There are cross allergies between two antithyroid drugs. They are also contraindicated in case of thyroid cancer TSH depends on severe blood or liver disease. They should also not be taken in case galactose intolerance, Lapp lactase deficiency or malabsorption syndrome glucose or of galactose. UTP is also contraindicated in case of vasculitis and fructose intolerance or sucrase/isomaltase deficiency. Carbimazole and thiamazole are also contraindicated during lactation, in case of goiter, cholestasis (stopping of bile flow) pre-existing and a history of bone marrow damage or acute pancreatitis caused by this drug.
What precautions for use with antithyroid drugs?
To monitor tolerance to treatment and avoid the occurrence of agranulocytosis (significant drop in white blood cells) or liver disease, blood tests (NFS, hepatic enzymes or haemostasis according to the antithyroid drug concerned) are to be carried out before starting treatment and then every week (or every 10 days depending on the drug) for the first 6-8 weeks of treatment. This monitoring can then be less frequent but must remain regular. In order to assess the effectiveness of the treatment, a dosage of free T4 is also to be carried out at 4 weeks as well as a regular dosage (every 3-4 months) of free T4 and TSH. In addition, in case of fever, angina or any other sign of infection, bruises, bleeding, mouth ulcers, malaise as well as in case of liver disease, the treatment must be stopped immediately, have a blood sample taken and notify your doctor. Antithyroid drug overdose can lead to hypothyroidism with increased volume (or appearance) of a goiter. Non-pregnant women of childbearing age should have effective contraception. During the first trimester of pregnancy, it is strongly recommended to favor UTP over carbimazole or thiamazole because of the risk of malformations that the latter are likely to cause in the baby. The PTU does not cause malformation. The use of an antithyroid drug during pregnancy can in some cases lead to goiter formation or hypothyroidism in the baby. Monitoring of the fetus must then be put in place. It is recommended not to breastfeed during treatment. If breast-feeding cannot be avoided, PTU is to be preferred because it passes very little into breast milk (< 1%), unlike other drugs. Antithyroid drugs can interact with certain medications. In case of takingan anti vitamin K, INR must be carefully monitored due to changes in thyroid hormone levels. Also, an increase in the effect of antithyroid drugs may occur when combined with a hypoglycemic sulfonamide (antidiabetic), hydantoin, iodine or iodide. Closer monitoring is required in the event of taken concomitantly with medicines likely to lower white blood cells.
