Radioactive iodine (RAI) has been in use for over 40 years to treat hyperthyroidism and thyroid cancer. There is common knowledge that the aim in hyperthyroidism is to produce normal thyroid functions or render the patient euthyroid. It is the choice in properly selected patients as it is safe, convenient, and there is rapid elimination of goiter and symptoms. In thyroid cancer, namely - papillary and follicular or well- differentiated ones, it ablates residual thyroid tissues not removed totally by surgery. This has, in all studies, improved life expectancy.
The most important aspect in the use of 1-131 for hyperthyroidism is to stress to the patient the need for constant follow-up and the recognition of the signs and symptoms of hypothyroidism which is inevitable. Recurrent hyperthyroidism and iatrogenic hypothyroidism are two problems which interact in such a way that trying to solve one leads to exacerbation of the other. Cure follows a logarithmic relationship to activity or absorbed dose, while the hypothyroidism follows a linear relationship. Even though we calculate point for point the administered dose (fixed or preferred dose) hypothyroidism still occurs. Dose calculation schemes have been discussed and it follows that the higher the dose is, the higher the percentage of cure is but the higher the chance is for eventual hypothyroidism.
Diagnosis of hypothyroidism is based on clinical history that the patient received RAI for toxicosis, plus signs and symptoms of weight gain, hoarseness, sleepy, sluggish, muscle cramps, fatigue, poor memory and concentration, dry skin, constipation, depression, menstrual irregularities as menorrhagia or amenorrhea and infertility. Physical examination would reveal bradycardia, non-palpable thyroid gland, slow speech, cool dry skin and delayed relaxation of deep tendon reflexes. Biochemical parameters would show as elevated TSH, low T4 and low T3.
Management of thyroid hormone replacement involves using levothyroxine sodium in increasing amounts especially in the elderly. A dose of 50-100 mcg is usually needed but bigger amounts can be given depending on the response and individual need. Special precaution is given for elderly, pregnant and cardiac patients. Monitoring of serum TSH or T4 is necessary every three to six months.
For thyroid cancer patients, the hypothyroid state when the patient is being prepared for therapy or whole body scan, seems to be the problem. In order to avoid such hypothyroid state, liothyronine or T3 is used after stopping the levothyroxine for two weeks and then two weeks of totally no thyroid hormone. Another convenient method is using recombinant TSH on Day I and Day 2 at 0.9 mg/ml then do the whole body scan after 1-131 administration. Thyrogen is approved for diagnostic iodine scan and thyroglobulin stimulation but not yet for therapeutic purposes. It is safe well tolerated and side effects are minimal such as nausea, headache and rashes or urticaria. Three to five days after RAI therapy, thyroid hormone can be started otherwise on pre-requirement for an effective therapy is high serum TSH of at least 30, or a hypothyroid state.