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Blood Tests Before Starting Anti-Thyroid Drugs

From The Merck Manual of Medical Information Second Home Edition, p.951, edited by Mark H. Beers. Copyright 2003 by Merck & Co., Inc., Whitehouse Station, NJ. Available at: Accessed 3/27/07
Antithyroid Drugs Propylthiouracil (PTU), Tapazole and Methimazole are antithyroid drugs (ATDs) used in the United States. These prescription medications are in a class of drugs called thionamides. Carbimazole/Neomercazole is an antithyroid drug which is used extensively in Europe and Australia. Carbimazole is converted into methimazole in the body.

Antithyroid drugs work to lower thyroid hormone levels by interrupting the production of thyroid hormone. ATDs also act as a mild immunosuppressant which lowers autoantibody levels. Dr. Alan Rubin in his book, Thyroid for Dummies states that these drugs offer the only truly curative treatment option.

Typically US doctors will prescribe ATDs for a period of 12-18 months, then withdraw the medication to see if remission has occured. Medical literature states that ATDs can be taken indefinitely as long as there are no adverse reactions. While the clinical perspective is treat with ATDs for only 12-18 months, practically speaking it can take much longer, as experienced and observed by Graves’ patients themselves.

It has also been our collective experience that many patients are prescribed ATDs to be taken once daily. We suspect that this once-a-day dosing is done to bring about better patient compliance with taking the medication however, the dosing intructions provided by the manufacturersfor each of the ATD medications state that this drug should be taken multiple times per day as the half life of these drugs is short.

Before starting an antithyroid drug, all patients should have these two labs tests done as a baseline for future comparison. A white blood cell count (WBC) and a liver panel should be done on all patients prior to intiation of ATD therapy.

All patients taking an ATD should be informed by their prescribing healthcare professional that if they develop a fever with sore throat (signs of neutropenia or agranulocytosis) or yellowing of the eyes or skin (signs of toxic hepatitis) they should contact their prescribing doctor immediately. These symptoms could be a sign of a serious adverse reaction to the drug and should be investigated by doing a white blood cell count (WBC) and a hepatic panel or liver function lab test. These serious adverse reactions are rare. The rate of agranulocytosis in a large scale study was 0.4% as reported by Thyroid Disease Manager and is reversible upon the cessation of the medication. They also state that the rate of toxic hepatitis is reported as being so low that routine monitoring is not necessary.

Adverse reactions most often happen within the first few months of taking the drug and are often dose dependent. Less serious adverse reactions include pruritis and urticara. In cases where pruritis and urticaria develop it is permissible to provide an antihistamine and continue using the drug to see whether the reaction subsides spontaneously, as it commonly does.

Our Community advocates proper ATD dosing and monitoring. Proper lab tests done at appropriate intervals, yields the best possible chance of remission of Graves’ disease. It has been our collective experience that one cannot assume that their healthcare provider is providing proper dosing and monitoring, therefore the patient must educate themselves in this area.

2003 Cooper Report
2005 Cooper Report
Antithyroid Drug Update by Elaine Moore
Thionamides in the Treatment of Graves’ Disease
Patient Information: Antithyroid Drugs


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