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The Journal of Clinical Endocrinology & Metabolism Vol. 89, No. 11 5867
Copyright © 2004 by The Endocrine Society

Letter to the Editor

Authors’ Response: Rapid Preoperative Preparation for Severe Hyperthyroid Graves’ Disease

Claudia Panzer and Lewis Braverman

Section of Endocrinology, Diabetes and Metabolism, Boston University Medical Center, Boston, Massachusetts 02118-2393

Address correspondence to: Lewis Braverman, MD, Evans Building, Room 201, Boston, Massachusetts 02118. E-mail: lewis.braverman{at}bmc.org.

To the editor:

Dr. Liel raised an interesting possibility in her letter (1). She observed a prolonged disappearance time of free T3 [half-life (t1/2), 14.9 d] in kinetic studies of levothyroxine withdrawal in patients with differentiated thyroid cancer on suppressive doses of levothyroxine (2). Her possible explanation for this observation is a continued compensatory production of T3 from T4 by peripheral deiodinases. Based on this observation, Dr. Liel is questioning our definition of euthyroidism, which is based on normalized total T3 values at the time of thyroidectomy in our hyperthyroid patients with Graves’ disease, prepared for thyroidectomy with potent inhibitors of deiodination of T4 to T3, iopanoic acid and dexamethasone, along with propylthiouracil or methimazole, when possible. Dr. Liel references a study by Hermann (3) on postoperative hormone kinetics after thyroidectomy that was similar to her experience showing a "prolonged free T3 t1/2." Hermann (3) found that in patients who underwent thyroidectomy during hyperthyroidism, the postoperative decline of free T3 was biphasic and rapid, and the t1/2 of the first and second phases were 2.6 ± 1.6 h and 110 ± 100 h, respectively. All patients had free T3 levels in the normal range on postoperative d 2. Similar to their observations, free T3 index, an indirect measure of free T3, in our study (determined by multiplying total T3 by the T3 resin uptake and dividing by 100) declined from 241 ± 27.9 (mean ± SE) to 58.1 ± 6.7 before surgery (mean, d 7). Therefore the t1/2 for the free T3 index was approximately 3.5 d. It should also be noted that potent inhibition of outer ring deiodinase will not only decrease deiodination of T4, but would also decrease deiodination of T3. Finally, as noted in our original manuscript (4), the serum T4 and free T4 levels will decrease more slowly not only due to the decrease of T4 deiodination, but also because of a decrease in hepatic uptake of T4 after iopanoic acid administration. Another possible explanation is that her patients were hypothyroid during some of the time of serum sampling, and this would result in a decrease in hepatic 5'deiodinase activity.

Received July 27, 2004.

References

  1. Liel Y 2004 Rapid preoperative preparation for severe hyperthyroid Graves’ disease. J Clin Endocrinol Metab 89:5866–5867 (Letter)[Free Full Text]
  2. Liel Y 2002 Preparation for radioactive iodine administrationin differentiated thyroid cancer patients. Clin Endocrinol (Oxf) 57:523–527[Medline]
  3. Hermann M 1998 Early relapse after operation for Graves’disease: postoperative hormone kinetics and outcome after subtotal, near-total, and total thyroidectomy. Surgery 124:894–900[Medline]
  4. Panzer C, Beazley R, Braverman L 2004 Rapid preoperative preparation for severe hyperthyroid Graves’ disease. J Clin Endocrinol Metab 89:2142–2144[Abstract/Free Full Text]




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