The side effects of thyroxine

Updated April 17, 2017

L-thyroxine is an oral thyroid replacement that's commonly marked under the brand names Cytomel, Levoxyl, Levothroid, Synthroid and Unithroid. L-thyroxine is used as a thyroid replacement therapy for people with hypothyroidism, which means that their thyroid gland does not secrete an adequate amount of thyroid hormones. The medication is also prescribed to treat or ward off conditions such as lymphocytic thyroiditis, euthyroid or thyroid goitre and specific types of thyroid cancer. As with other medications, there are a number of potential side effects attributed with its usage.

Common Side Effects

Common side of L-thyroxine include changes in appetite, anxiety, excessive sweating, fatigue, headache, hyperactivity, insomnia and weakness. Seeing as the medication is a replacement therapy, many of the side effects serve as an indication that the dosage needs to be modified. For example, symptoms of increased metabolic activity (muscle cramps, palpitations, tremors, stomach disturbances and restlessness) are a sign that the dosage is too high.


The side effects of the medication are closely linked to the prescribed dosage, which differs depending on the patient's age, weight, thyroid condition, pre-existing medical conditions, other medications administered, tolerability and response to the medication and its effectiveness. That being said, many of the side effects decrease or are completely eliminated once the dosage is stabilised.

Drug Interactions

L-thyroxine has the potential to adversely affect or alter a number of other prescription and non-prescription drugs. Notify your doctor of any medications, herbal supplements or other regimens being taken, especially decongestants, stimulants, diabetic remedies and hormonal supplements or birth control pills. L-thyroxine can also alter the potency and effectiveness of certain anticonvulsant medicines, blood thinners, beta blockers, cholesterol-lowering drugs and heart medications.


Since particular medicines can reduce the body's ability to absorb L-thyroxine, doctors usually advise patients to take things like iron supplements, calcium supplements and calcium antacids at least four hours from the time they take L-thyroxine. Additionally, prior to starting L-thyroxine, make your doctor aware of any pre-existing medical conditions or allergies you have, particularly adrenal gland problems, diabetes and cardiovascular problems. It's worth noting that drugs like L-thyroxine are not intended to be used for weight control purposes.


L-thyroxine is measured as having a long half-life, which means that it may take up to 4 to 6 weeks before the medication is able to reach its maximum level of effectiveness. Individuals taking the medication may be asked to have their blood drawn at specific intervals to monitor their levels and thyroid function and to assess the medication's current dosage within their system. Individuals who are pregnant or plan on becoming pregnant are advised to consult with their doctor prior to breast-feeding, as small amounts of L-thyroxine get excreted into breast milk.

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About the Author

Serena Spinello holds two master’s degrees and is pursuing her Ph.D. in medical science. She has been a professional writer and researcher for over 10 years and is an active member of the American Medical Writers Association, Academy of Medical Educators, and the National Association of Social Workers.