TORONTO — The risk of provoking thyroid storm with administration of radioactive iodine appears to be vanishingly small even in cases of severe thyrotoxicosis, Vani Vijayakumar, M.D., said at the annual meeting of the Society of Nuclear Medicine.
Patients undergoing such treatment therefore do not need to be placed on thiouracil drugs first, provided that they receive concomitant β-blocker therapy and good counseling, added Dr. Vijayakumar of the nuclear medicine section at the University of Texas Medical Branch, Galveston.
In a retrospective study, Dr. Vijayakumar and her colleagues identified 122 patients who were treated for severe thyrotoxicosis between August 2003 and December 2004. Patients were judged to have severe hyperthyroidism when there were marked signs and symptoms of thyrotoxicosis, suppressed levels of TSH, and markedly elevated levels of free T4 or free T3.
The diagnosis of severe thyrotoxicosis also required radioactive iodine (I-131) uptake exceeding 30% at 4 or 24 hours after administration of I-131. Of the 122 patients identified with severe thyrotoxicosis, the diagnosis of Graves' disease predominated. For the group overall, TSH levels were between 0.01 and 0.06 mIU/L.
Most of the patients were females between ages 15 and 64 years, and the range of radioactive iodine uptake was between 31% and 92%. All patients were treated with 10–20 mCi of I-131 and were evaluated for any evidence of thyroid storm.
“Ninety-two patients had one radioactive iodine treatment, 21 patients had two radioactive treatments, and only a few had three treatments,” Dr. Vijayakumar reported. Eight patients received no radioactive iodine treatment at all. Four patients were thyrotoxic and were placed on propylthiouracil (PTU); three patients were already on PTU and did not receive I-131.
Most patients were placed on β-blocker drugs at the time of initial I-131 therapy, and those drugs were continued for at least 2 months. Patients were educated about the signs and symptoms of thyroid storm before receiving I-131 treatment, Dr. Vijayakumar said.
At first follow-up 2 months later, “none of these patients had any symptoms of thyroid storm,” Dr. Vijayakumar said. In a subset of 39 high-risk patients—defined as having I-131 uptakes in excess of 70%, marked signs and symptoms of hyperthyroidism, and markedly elevated free T4 or free T3 levels—I-131 treatment was well tolerated, and led to “marked clinical improvement.” Again, none of these high-risk patients had any sign of thyroid storm.
“Thyroid storm after radioactive iodine is extremely rare,” Dr. Vijayakumar concluded. “Hence, it is safe to treat these hyperthyroid patients in thyrotoxicosis with radioactive iodine, [although] simultaneous β-blockers are necessary, and patient education is also important. With all these measures in place, 4–6 weeks of prior medical treatment [with PTU] may not be necessary.”
Thyroid storm, an acute, life-threatening thyroid hormone-induced hypermetabolic state that can occur in patients with thyrotoxicosis, is usually precipitated by stress such as surgery or infection.
Although thyroid storm is rare—occurring in 1%–2% of hyperthyroid patients—mortality approaches 20% if the condition goes unrecognized and untreated. Features of thyroid storm include a high fever, flushing, sweating, tachycardia, agitation, and delirium. The diagnosis of thyroid storm is largely clinical as thyroid function tests cannot differentiate between thyroid storm and thyrotoxicosis.