Tumors secreting human chorionic gonadotropin (molar pregnancy, choriocarcinoma) a similar clinical picture include toxic adenomas and toxic multinodular goiter. A radioactive iodine uptake thyroid scan may help differentiate these two conditions from Graves disease. Rare conditions resulting in thyrotoxicosis include metastatic thyroid carcinoma causing thyrotoxicosis, amiodarone induced thyrotoxicosis, iodine induced thyrotoxicosis, postpartum thyroiditis, a TSH-secreting pituitary adenoma, an hCG-secreting choriocarcinoma, struma ovarii, and “de Quervan’s” or subacute thyroiditis (394). Factitious ingestion of thyroxine or desiccated thyroid should be considered in patients with eating disorders. Patients with thyrotoxicosis factitia demonstrate elevated T3 and T4, suppressed TSH, and a low serum thyroglobulin level, whereas other causes of thyroiditis and thyrotoxicosis demonstrate high levels of thyroglobulin. Potential causes of hyperthyroidism are listed in Table 31.14.
Iodine-131 Ablation Treatment of women with hyperthyroidism of an autoimmune origin presents unique challenges to the physician who must consider the patient’s needs and her reproductive plans. Because the drugs used to treat this disorder have potentially harmful effects on the fetus, special attention must be given to the use of contraception and the potential for pregnancy.
A single dose of radioactive iodine-131 is an effective cure in about 80% of cases and is the definitive treatment in nonpregnant women. Any woman of childbearing age should be tested for pregnancy before undergoing diagnostic or therapeutic administration of iodine. Ablation of a second-trimester fetal thyroid gland and congenital hypothyroidism (cretinism) from treatment during the first trimester were reported (395). Nuclear medicine professionals provide expertise in the administration of the radioactive isotope, and because the effect of the radioactive iodine is not immediate, the endocrinologist continues to provide suppressive medical treatment for 6 to 12 weeks after administration of iodine while the patient remains hyperthyroid. As early as 2 to 3 months after treatment, patients may become hypothyroid and should be supplemented with thyroxine as indicated by serum levels of free thyroid hormone levels (368). TSH testing is not sensitive for predicting thyroid function during this time as changes in TSH lag 2 weeks to several months behind thyroid function changes (368). Failure to respond to iodine 6 months after treatment may require a repeat treatment with radioactive iodine (394). Postablative hypothyroidism develops in 50% of patients within the first year after iodine therapy and in more than 2% of patients per year thereafter.
A higher rate of miscarriage was noted in women treated with iodine therapy in the year preceding therapy, but there is no reported increase in the rate of stillbirths, preterm birth, low birth weight, congenital malformation, or death after therapy (396). Many thyroidologists and nuclear medicine specialists are willing to allow pregnancy earlier than 1 year after therapy if patients receive replacement therapy with levothyroxine.