A large number of medicines are available, generally not requiring a prescription, to treat the cough or symptoms of a cold or flu. Many of these medicines carry warnings about potential problems if the patient has thyroid disease. These medicines can be subdivided generally into two groups; those that contain iodine or those that contain adrenergic agonists (stimulants) such as ephedrine, norepinephrine, pseudoephedrine, terbutaline, adrenaline, or xylometazoline.

Iodine is taken up and used by the thyroid to make thyroid hormone. In patients with untreated hyperthyroidism, administration of iodine can make the hyperthyroidism worse. However, once treatment with medications such as PTU or methimazole has been started, excess iodine is blocked from being incorporated into the thyroid hence cough and cold medicines are usually safe to take. Small amounts of iodine may precipitate hyperthyroidism or hypothyroidism in previously normal patients, however this is also uncommon. Patients already taking thyroid hormone have a very low risk of having any new thyroid-related problems develop if they take cough and cold medicines containing iodine. For more on how iodine may affect the thyroid, see Iodine.

Medicines containing ingredients such as ephedrine, pseudoephedrine, adrenaline, noradrenaline, or xylometazoline (adrenergic agonists) are often used to constrict dilated blood vessels in those, in order to relieve sinus or nasal congestion, reduce stuffiness and improve breathing. These medicines if taken in sufficient doses, may also increase the heart rate and blood pressure. Similarly, patients taking medicines for asthma that contain adrenaline, salbutamol (Ventolin), terbutaline or related drugs may also note an increase in heart rate, nervousness and tremor. Since patients with moderate to severe hyperthyroidism may experience palpitations, tremor, increased sweating, rapid heart beats, and increased blood pressure, the class of cough and cold medicines that contain adrenergic agonists should be used with caution under a physicians supervision until the hyperthyroidism is brought under control.

Patients who have had complete removal of their thyroid and who are already taking thyroxine do not need to be as concerned about taking medications containing small amounts of iodine or adrenergic stimulants.