I very much appreciate your answers to my questions. They have been quite helpful. May I ask you another clinical question? Would you recommend lowering the dose of thyroid hormone prior to thyroid surgery in patients with thyroid cancer who have suppressed TSH, normal FT4? Also, when do your recommend restarting thyroid hormone therapy in a patient who has received hi doses ie 150 mci of I131 Rx who will have a whole body scan 1 week post treatment?
Although such patients are by definition chemically thyrotoxic, usually they receive something like 125% of the “normal” T4 daily requirement, not the 200-1000 % excess seen in spontaneous thyrotoxicosis. I believe that in most cases there is no need to be concerned. Obviously if the patient has a history of heart disease or arrhythmias, the treatment would already be different and more caution would be required- that is, use of beta blockers, reduction in T4 dose. After administration of the therapeutic 131-I to a patient who has become hypothyroid and has an elevated TSH, the logical goal is to keep the absorbed iodine in the thyroid tissue or tumor, which should be fostered by decreasing iodide turnover in the thyroid tissue if possible. For that reason it seems logical to promptly suppress TSH, and I restart T4 treatment with (usually) double the prior dose (for 3 days) beginning 24 hours after treatment is given. I know that some MDs wait longer, but do not understand why. One could make a case for beginning treatment immediately after administering the 131-I, but I have never pursued that approach.
L De Groot,MD