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Propylthiouracil may be more likely to cause agranulocytosis than carbimazole
The background of the study. There are three antithyroid
drugs—carbimazole, methimazole, and propylthiouracil. Carbimazole
itself is inactive, but it is rapidly converted to methimazole.
The most serious adverse effects of these drugs are agranulocytosis
(white-cell count <0.5x109 cells/L) and granulocytopenia (>0.5
to <2.0x109 cells/L), but data concerning their frequency are
sparse. In this study, the frequency of agranulocytosis and granulocytopenia
was determined based on spontaneous reporting.
How the study was done. Since 1963, physicians in the United Kingdom have been reporting adverse drug effects to the Committee on Safety of Medicines. All reports of agranulocytosis and granulocytopenia associated with carbimazole and propylthiouracil (methimazole is not available) from 1963 to 2003 were reviewed. The number of prescriptions for each drug dispensed from 1981 to 2003 was obtained from the Department of Health.
The results of the study. During the study period, there were 94 reports of agranulocytosis, of which 18 (19 percent) were fatal, and 85 reports of granulocytopenia (2 fatal [2 percent]) in patients treated with carbimazole. Among patients treated with propylthiouracil, there were 12 cases of agranulocytosis (1 fatal [8 percent]) and 14 cases of granulocytopenia (1 fatal [7 percent]).
From 1991 to 2003, when more detailed information was available, there were 54 reports of agranulocytosis (45 with carbimazole, 9 with propylthiouracil) and 63 reports of granulocytopenia (51 with carbimazole, 12 with propylthiouracil), with 6 deaths. The median dose of carbimazole was 40 mg daily, (range, 10 to 80) and that for propylthiouracil 300 mg daily (range, 50 to 1200). The median duration of treatment was 31 days (range, 14 to 105) for carbimazole and 33 days (range, 17 to 75) for propylthiouracil.
Based on 5.23 million prescriptions dispensed from 1981 to 2003, the respective rates of agranulocytosis during this interval were 13 and 38 per 1 million prescriptions for carbimazole and propylthiouracil, and the respective rates for granulocytopenia were 16 and 42 per 1 million prescriptions.
The conclusions of the study. Based on spontaneous reporting, on a per-prescription basis, carbimazole is less likely to cause agranulocytosis or granulocytopenia than is propylthiouracil.
The original article. Pearce SH. Spontaneous reporting of adverse reactions to carbimazole and propylthiouracil in the UK. Clin Endocrinol (Oxf) 2004;61:589-94.
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