ICM are iodine-rich drugs administered in significant quantities during radiological investigations, well in excess of the recommended daily amount of iodine, equivalent to an acute dose.The daily iodine intake recommended by the World Health Organization is 150 µg/day (1). The prevalence of ICM-induced thyroid dysfunction varies from 1 to 15% according to recent studies (2).

Side effects

The excess of iodine can cause acute thyroid dysfunction, particularly in patients at risk. It can lead to hyperthyroidism or hypothyroidism, with numerous complications, such as arrhythmias, neuropsychiatric problems or coronary disease. Repeated ICM exposure (>1 time of ICM per year) increases significantly the risk of these thyroid disorders (3).

Pathophysiology

In case of iodine overload, such as ICM injection, the thyroid has the capacity to adapt by reducing the organification of iodine, and thus the production of thyroid hormones. It is known as the Wolff-Chaikoff effect. After a while, this mechanism is reversed, allowing hormone production to take over. ICM-induced thyroid dysfunction is explained by a failure of these mechanisms.

Hyperthyroidism

Thyrotoxicosis is generally a very late adverse reaction which occurs more than a week after contrast agent injection (4). It is predominantly found in regions with iodine deficiency and in patients with underlying nodular goiter or latent Graves’ disease (2). It may lead to life-threatening complications.

Hypothyroidism

ICM-induced hypothyroidism is less common. It occurs in patients with autoimmune thyroiditis, living in areas with sufficient iodine supply (2).

Pregnancy and lactation

Risks of using ICM on pregnant women must also be considered. CT scan remains the imaging of choice in several cases during pregnancy; top of the list, thoracic angioscan for suspected pulmonary embolism. Then, there is a risk of neonatal hypothyroidism induced by ante-natal exposure to ICM after 12 weeks of pregnancy. The prevalence of congenital hypothyroidism turns around 1/4000 births (5). It represents the main etiology of treatable mental disability.

With regard to lactation, breast feeding may be continued normally when ICM is given to the mother, with no risk.

Contraindications

The main contraindication of ICM is manifest hyperthyroidism with obvious clinical symptoms. In case of emergency, treatment with antithyroid drug should be initiated prior to ICM administration.

ICM can be used in case of hypothyroidism without proven risk.

Radioactive iodine and isotope imaging of the thyroid should be avoided for at least 2 months after ICM administration.

Management

Before ICM administration

The general principle of giving doses of ICM that are "as reasonable as possible" must be observed. The palpation of the neck to detect goitres, and thus identify patients at greater risk is suggested. Some experts suggest drug prophylaxis (with methimazole and/or perchlorate) prior to ICM injection, in selected cases.

Baseline thyroid function testing of the general population before ICM exposure is not recommended (2). A normal baseline thyroid function does not exclude thyroid dysfunction after ICM administration.

European Thyroid Association recommends measurement of baseline serum TSH in high-risk patients for ICM-induced thyroid disorders, as well as thyroid hormones, if serum TSH turns out abnormal.

After ICM administration

In the absence of prospective studies in the management of ICM-induced thyroid disorders, a personalized approach based on patient’s age and history, clinical signs and iodine intake must be established.

In patients at high risk, especially in the elderly and those with cardiovascular diseases, close monitoring of thyroid function after exposure is recommended.

In this particular cases, treatment of ICM-induced hyperthyroidism with antithyroid drugs should be considered for severe and prolonged disorder.

In ICM-induced hypothyroidism, temporary levothyroxine may be considered in symptomatic patients, with an autoimmune thyroiditis and in women planning pregnancy (2).

Patients on thyroid hormone replacement are not at risk for ICM-induced thyroid dysfunction and do not require specific management.

Despite the fact that the exposure to a single high dose of intravenous iodine is unlikely to have a clinically significant effect on thyroid function at birth (5), all newborns with ante-natal ICM exposure are screened for congenital hypothyroidism during the first week (4), so that early initiation of replacement therapy is set up, if an abnormality is detected.