Thyrotoxicosis radioactive iodine

Different forms of radioiodine have been used at different times, including 123I, 12SI, and 131I. Radioactive iodine is used to scan the thyroid gland and in the treatment of thyrotoxicosis. See in the monograph on Radioactive iodine. 

Best JD, Chan V, Khoo R, Teng CS, Wang C, Yeung RT. Incidence of hypothyroidism after radioactive iodine therapy for thyrotoxicosis in Hong Kong Chinese. Clin Radiol 1981 32(1) 57—61. 

Hoogenberg K, Beentjes JA, Piers DA. Lithium as an adjunct to radioactive iodine in treatment-resistant Graves thyrotoxicosis. Ann Intern Med 1998 129(8) 670. 

Serial measurement of Tg is most useful in detecting recurrence of DTC following surgical resection or radioactive iodine ablation. Tg determination is used as an adjunct to ultrasound and scanning. Assessment of serum Tg also aids in the management of infants with congenital hypothyroidism, All patients with hyperthyroidism should have elevated Tg low concentrations of Tg may be an indication that thyrotoxicosis factitia is present. 

Ross DS. Syndromes of thyrotoxicosis with low radioactive iodine uptake. Endocrino Metab Clin North Am 1998 27 169-185. 

Radioactive iodine is also used to treat thyrotoxicosis and to assess thyroid function. Eventually it causes hypothyroidism, which has to be treated by thyroid hormone replacement. 

Radioactive iodine, although not a hormone itself, is taken up by the thyroid gland and incorporated into thyroid hormones. During this process, it emits radiation that causes destruction of the thyroid gland. It is used specifically to treat thyroid tumours. Radioactive iodine is also used to treat thyrotoxicosis (Chapter 6). 

Thyrotoxicosis occurs in about 0.2% of pregnancies and is caused most frequently by Graves disease. Antithyroid drugs are the treatment of choice radioactive iodine is clearly contraindicated. Historically, propylthiouracil has been preferred over methimazole because transplacental passage was thought to be lower however, both propylthiouracil and methimazole cross the placenta equally. Current data suggest that either may be used safely in the... 

The most severe form (f hyperthyroidism is thyroid storm, a rare but life-threatening disease that usually is precipitated by an intercurrent medical problem. Precipitating factors include infections, stress, trauma, thyroidal or nonthyroidal surgery, diabetic ketoacidosis, labor, heart disease, and rarely, radioactive iodine ablation. Clinical features are similar to those of thyrotoxicosis, but are more severe. Cardinal features include fever and tachycardia out of proportion to the fever. Nausea, vomiting, diarrhea, agitation, and confusion are frequent presentations. The abnormalities in TFTs are not necessarily worse than those in uncomplicated thyrotoxicosis and thyroid storm is a clinical diagnosis. 

The major therapies available for Graves disease are surgery, thyroid-suppressant drugs, and radioactive iodine in sufficient dosage to destroy the gland. Ipodate—an iodine-containing x-ray contrast material—and beta-blockers are of value in severe thyrotoxicosis. 

W8. Wellby, M. L., Kanagasabapthy, A. S., Marshall, J., Higgins, B., and Graycar, L. E., The pattern of serum thyroxine, triiodothyronine and thyrotrophin in patients treated with radioactive iodine for thyrotoxicosis. Endocrine Soc. Aust., Proc. 17th Annu. Meet. p. 29 (1974). 

The management of amiodarone-induced thyrotoxicosis has been extensively reviewed [42 ]. For type 1 thyrotoxicosis, thionamides are the best treatment (possibly associated with potassium perchlorate) for type 2, glucocorticoids are the treatment of choice. However, if rapid restoration of euthyroidism is necessary, a short course of iopanoic acid followed by total thyroidectomy is an option. Radioactive iodine has a marginal role. The authors addressed some important controversies, practical problems, and unanswered questions. 

What to do if amiodarone (in the event of withdrawal) needs to be given again This is not infrequent. In type 1 thyrotoxicosis, prophylactic thyroid ablation with radioactive iodine is recommended, whereas in type 2 an expectant strategy is suggested. 

Dr. Braverman told us that iodine-induced thyrotoxicosis could be diagnosed by measuring urinary iodine excretion levels. I would like to comment on this point. If the iodine contamination is of recent onset, urinary iodine will of course be elevated. However, several studies have indicated that after coronary arteriography for instance, iodine in urine will be elevated up to two weeks later. After that period it will rapidly come down to normal excretion levels. My other comment concerns the lowering of thjn-oidal radioactive iodine uptake in patients contaminated with iodine. In some patients with iodine-induced thyrotoxicosis, the radioactive iodine uptake remains elevated. Therefore, a high uptake does not exclude the diagnosis of iodine-induced thyrotoxicosis. 

I think Dr. Jonckheer is correct. However, if you take 100 patients with iodine-induced hyperthyroidism, at least in the US, I would guess that 95 % of them have a suppressed radioactive iodine uptake. As a matter of fact, in a joint study with Dr. Pinchera on iodine-induced thyrotoxicosis in patients treated with amiodarone, we showed that the condition could be associated with normal or elevated uptakes. Those cases represent, however, a minority. The majority of patients with Jod-Basedow in the US will have an elevated urinary iodine excretion and a low radioactive iodine uptake. 

Iodine, which is selectively accumulated by the thyroid gland, provides another good example of selective action through specific distribution. This process is easily followed with radioactive iodine ( T, half-life 8 days), which is used for the treatment of thyrotoxicosis. Depending on the dosage, the radioactive chemical can merely inhibit the excessive metabolism of the gland or actually attack a tumour in it. The usual oral dose is only I0" g, yet 80 per cent of this can be demonstrated in the gland soon after administration. 

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