sodium iodide I-131 (Rx)

Frequency Not Defined

Reported with doses for treatment of benign disease (eg, hyperthyroidism)

• Hypersensitivity reactions
• Increased radiation accumulation
• Fetal toxicities
• Exposure to breast tissue with lactation
• Transient infertility
• Gastrointestinal disorders: Sialadenitis
• Cardiac disorders: Chest pain and tachycardia
• Skin and subcutaneous tissue disorders: Iododerma, itching skin, rash, and hives
• Endocrine disorders: Hypothyroidism, hyperthyroidism, thyrotoxic crisis, hypoparathyroidism
• General disorders and administration site conditions: Local swelling

Reported with doses for treatment of malignant disease

• Blood and lymphatic system disorders including fatalities: Radiation sickness, bone marrow depression, anemia, leucopenia, thrombocytopenia, and blood dyscrasia
• Neoplasms benign, malignant and unspecified (including cysts and polyps): Leukemia and solid cancers
• Eye disorders: Lacrimal gland dysfunction
• Gastrointestinal disorders: Salivary gland dysfunction, nausea, vomiting
• Congenital, familial and genetic disorders: Congenital hypothyroidism and chromosomal abnormalities
• Cerebral Edema (patients with iodine-avid brain metastases)
• Radiation pneumonitis (patients with iodine-avid lung metastases)
• Pulmonary fibrosis (patients with iodine-avid lung metastases)

Postmarketing Reports

Eye disorders: Dacryostenosis acquired

Contraindications

Therapeutic and diagnostic use

• Women who are pregnant or may become pregnant; defer use in women of childbearing age until the possibility of pregnancy has been ruled out
• Breastfeeding

Therapeutic use

• Vomiting and diarrhea
• Treatment of thyroid malignancies shown to have no iodide uptake, which include the majority of medullary and anaplastic carcinomas
• Patients receiving concurrent anti-thyroid therapy

Cautions

Diagnostic use

• Recent intake of stable iodine in any form, or the use of thyroid or anti-thyroid drugs will affect the uptake of radioiodide

Therapeutic use

• Consider pre-treatment anti-thyroid medication to help deplete thyroid hormone; discontinue anti-thyroid therapy three days before administration of sodium iodide I 131; consider a beta-blocker pre or post-treatment to minimize risk of hyperthyroidism and thyroid storm
• Instruct patients to follow radiation safety precautions after receiving therapy to minimize radiation contamination of other persons or the environment; patients should avoid close contact with others, especially pregnant women and children, and take care to avoid contamination of other persons or the environment with body fluids
• Thyroiditis may cause gland enlargement resulting in tenderness and swelling of neck, pain on swallowing, sore throat, and cough; which may occur approximately the third day after sodium iodide I 131 administration. Consider management with pain-reliever or anti-inflammatory medications
• Hypersensitivity reactions, including rash and hives reported
• Transplacental passage of sodium iodide I-131 can cause severe and possibly irreversible hypothyroidism in neonates (see Contraindications)
• Transient dose-related impairment of testicular function in men and transient ovarian insufficiency in women has been reported after sodium iodide I 131 therapy; consider sperm banking for men who are anticipated to receive cumulative sodium iodide I 131 doses greater than 19,000 MBq (520 mCi)
• Therapy contributes to a patient’s overall long-term cumulative radiation exposure, which is associated with increased risk of cancer; follow safe handling and administration to minimize radiation exposure to patient and healthcare providers

• Radiation-induced toxicities

  • Dose-dependent fatalities (bone marrow suppression, malignancy)
  • Dose-dependent hematopoietic suppression which manifests as a transient thrombocytopenia or neutropenia 3-5 weeks following sodium iodide I 131 administrations, may lead to increased susceptibility to infections or bleeding
  • Salivary gland toxicity: sialadenitis, xerostomia
  • Lacrimal gland toxicity: conjunctivitis, xerophthalmia, and epiphora
  • Obtain complete blood count within one month of therapy; if patients show leukopenia or thrombocytopenia, use dosimetry to determine a safe sodium iodide I 131 activity, while delivering less than 2 Gy to bone marrow
  • Advise good hydration for one week following sodium iodide I 131 administration and stimulate salivary flow via a sialagogue (e.g. sugar-free candy or gum, pilocarpine, and ascorbic acid) to reduce radiation exposure to salivary glands
  • Advise patients to void frequently after administration of radioiodide to enhance excretion

Drug interaction overview

• Concomitant use of bone marrow depressants may enhance depression of the hematopoietic system caused by the use of large doses of sodium iodide I 131
• Certain food or drugs may alter thyroid uptake of sodium iodide I 131 and diminish effectiveness; recent intake of stable iodide in any form, or the use of thyroid or anti-thyroid drugs may diminish thyroid uptake of sodium iodide I 131
• Review the patient’s history, current medications, and recent diagnostic tests prior to administration of sodium iodide I 131
• Advise patients to maintain a low-iodide diet two weeks prior to radioiodide administration and continue for several days during the uptake or imaging process and to discontinue taking amiodarone, topical iodide, kelp, agar, carrageenan, Lugol solution, thionamide medications (propylthiouracil, methimazole, carbimazole), multivitamins containing iodide, natural or synthetic thyroid hormones, iodide containing foods, iodized salt, dairy products, egg yolks, seafood, turkey and liver, before they undergo the procedure for the periods described in the prescribing information

Pregnancy

Contraindicated in pregnancy because fetal exposure can lead to neonatal hypothyroidism, which in some cases is severe and irreversible

Data from published literature describe reports of neonatal thyroid abnormalities after fetal exposure; including agenesis of thyroid and hypothyroidism; no animal reproductive studies have been conducted

A fetus exposed to sodium iodide I 131 can develop neonatal hypothyroidism; delay in diagnosis of neonatal hypothyroidism after exposure to sodium iodide I 131 in utero can result in severe sequelae such as cognitive impairment and delayed bone age; monitor thyroid function in any infant born after in utero exposure to sodium iodide I 131

Pregnancy Testing

• Obtain a pregnancy test in females of reproductive potential and verify absence of pregnancy within 24 hours prior to administration of treatment

Contraception

• Advise females and males of reproductive potential to use effective contraception during treatment and for at least six months after last dose

Infertility

• Females: Fertility may be impaired with treatment; transient amenorrhea and ovarian insufficiency observed after sodium iodide I 131; therapy in females; the literature describes reports of transient menstrual cycle irregularities, including amenorrhea, and ovarian failure in females treated with cumulative doses of 1,000 MBq to 59,000 MBq (27 mCi to 1,595 mCi) sodium iodide I 131; in a published literature analysis, the effects on fertility occurred in up to 30% of women treated with sodium iodide I 131, and may resolve 12 months after treatment.
• Males: Fertility may be impaired with treatment; discuss sperm banking for males who are expected to receive a high cumulative dose of sodium iodide I 131; transient dose-related impairment of testicular function after sodium iodide I 131 therapy has been reported in the published literature; the literature describes reports of males treated with sodium iodide I 131 at doses of 370 MBq to 22,000 MBq (10 mCi to 595 mCi) resulting in transiently impaired testicular function (including spermatogenesis); the risk of persistent testicular dysfunction increases after administration of repeated or high cumulative radioiodide exposure

Lactation

Contraindicated in lactating women because sodium iodide I 131 concentrates in breast via increased expression of sodium iodide symporter in breast tissue with lactation and can lead to hypothyroidism in the infant through breastfeeding

Advise lactating women to discontinue breastfeeding at least 6 weeks prior to administration of sodium iodide I 131 to allow sufficient time for involution to occur and to avoid excess concentration of sodium iodide I 131 in breast tissue

Consider administration of drugs to suppress lactation; consider diagnostic scintigraphy before administration of sodium iodide I 131 to assess the persistence of uptake by breast tissue; if sodium iodide I 131 is administered in postpartum period, the lactating mother should not breastfeed the infant

If sodium iodide I 131 is administered postpartum, breastfeeding should not be restarted for the remainder of the postpartum period

Infants exposed to sodium iodide I 131 through breast milk are at risk for development of hypothyroidism because sodium iodide I 131 is distributed into breast milk and may reach concentrations equal to or greater than concentrations in maternal plasma

Pregnancy Categories

A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.

B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.

C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.

D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.

X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.

NA: Information not available.

Mechanism of Action

Radiopharmaceutical

Thyroid ablation: Destruction of thyroidal tissue is achieved by the beta emission of sodium iodide I-131 and a subsequent thyroid-specific inflammatory response, causing fibrosis and destruction of the thyroid over weeks to many months

Diagnostic: Shows thyroid iodide uptake as a measure of thyroid function

Absorption

Readily absorbed from GI tract after oral administration

Distribution

Primarily distributed within the extra-cellular fluid of the body It is trapped by the thyroid

Thyroid uptake of iodide is usually increased in hyperthyroidism and in goiter with impaired hormone synthesis, decreased in hypothyroidism, and normal-to-decreased in hypothyroidism receiving iodine

It is also concentrated by the stomach, choroid plexus, and salivary glands

Protein bound: None

Metabolism

Trapped iodide is oxidized to iodine and organically incorporated so rapidly that the iodide trap of the thyroid contains <0.2 % free iodide in comparison to the organically bound iodine

Elimination

Half-life: 8.04 days; decays by beta emission and associated gamma emission

Excretion: Primarily via kidneys; normal range of urinary excretion is 37-75 % of the administered dose, varying with the thyroid and renal function