Brief introduction
Principle
Various aspects of normal thyroid function, and increased thyroid function and its effects on the human body
have already been covered in this page on pertechnetate thyroid scan. Please go through it for better understanding of the
content that follows.
131I / iodine-131 (radioiodine) therapy
Iodine is an element similar to chlorine and fluorine (halogen), and is commonly found in packaged salt,
Betadine® ointment, Iodex® balm, contrast agents used for X-ray and CT scans, seafood, etc. As discussed in
the above page, it is essential for synthesis of the thyroid hormones T3 and T4 by the thyroid cells
(thyrocytes). Iodine-131 (or 131I) is a radioactive form of iodine, which emits beta particles.
Beta particles are essentially electrons travelling at very high speeds, and are a type of particulate radiation. They damage the
DNA of thyrocytes, which prevents them from replicating and eventually causes their death. Thus, the final
effect of a sufficient dose of radioiodine is absent or markedly reduced production of thyroid hormones in
the body. Hence, radioiodine therapy (RAI) is one of the treatment options for hyperthyroidism /
thyrotoxicosis (in addition to anti-thyroid drugs and surgical removal of thyroid).
Radiation safety aspects
After administration, iodine-131 tends to be be in detectable quantities in the body for a few (2 - 3)
weeks. Apart from beta particles, it also emits gamma
rays, which tend to be penetrating in nature, and hence have much lesser probability of interacting
with human cells. Beta particles on the other hand can travel for very short distances (about a millimetre),
but have much stronger tendency to interact with (and damage) cells. So, iodine-131 does not have any
remarkable effect when outside the body. Also, it does not have any tangible effect in very low doses.
Iodine is eliminated from the human body in stool, urine, sweat, saliva, tears and milk. Hence, the most
important guiding principle of radiation safety in context of radioiodine therapy is to prevent /
minimize physical contact with the patient and their bodily fluids. However, the actual amount of
iodine-131 in sweat, saliva and tears is too little to warrant anxiety.
Indications (conditions in which advised)
- Severe cases of thyrotoxicosis wherein the anti-thyroid drugs (ATDs) prove insufficient.
- Adverse effects with ATDs.
- Relapse of thyrotoxicosis following initial benefit from ATDs.
- Erratic response to ATDs.
- If the patient using ATDs is unable to follow up regularly with their treating doctor.
- Relapse following earlier treatment with radioiodine therapy.
- Preparation for surgery of very bulky thyroid glands (goitre).
What to expect as part of procedure?
Important steps
- A medical doctor's written advice to carry out RAI therapy is a must.
- Patient registration.
- In-person briefing about the procedure, drug and food modifications, radiation precautions and follow up
period.
- Consent.
- 2 to 3 capsules are to be swallowed with water. Radiation levels of neck and stomach regions are
measured to ascertain capsules' reaching the stomach.
- Patient is observed during 2 hours. During this period, some water can be had.
- At the end of 2 hours, radiation levels of neck, stomach and entire body (from 1 metre away) are
measured.
- A summary mentioning dose of ingested RAI, radiation levels and general instructions is issued to the
patient.
Discomfort / risks
The following can happen within few hours of RAI ingestion. They are typically mild, self-limited, and do not
require any specific treatment.
- Altered taste.
- Burning sensation in stomach / nausea.
Time taken for the entire patient visit
- Typical: 3.5 hours.
- Be prepared for: 5 hours.
What to expect after procedure?
Precautions / special care
Medical aspects
- Fasting. Continue fasting for 4 hours after ingesting the RAI capsules (drinking water after
initial 1 hour is acceptable). This helps absorption of iodine-131. Your first meal after RAI therapy
should consist of items that are unlikely to cause heart burn or nausea (avoid excessively spicy and
oily food). Also, do not have beverages (tea, coffee or cola) on an empty stomach. If vomiting occurs
within a few hours of ingesting RAI capsules, total amount of RAI absorbed would be reduced with
attendant risk of incomplete destruction of the thyroid gland and subsequent relapse of hyperthyroidism.
- Continue iodine restrictions for 5 days following RAI therapy. In particular, avoid seafood, Iodex® and
iodized salt (please see the next point).
- For 5 days following RAI therapy, have non-iodized salt as part of diet. If non-iodized salt is
unavailable, iodized salt can be used, but it is to be added to the meal while being cooked (high
temperature tends to vapourise iodine). Salt is vital for health, and must not be completely
eliminated from the diet.
- Continue to have all other medications (especially propranolol).
- Consult your referring doctor after 4 to 6 weeks from RAI therapy (and earlier, if directed by them). As
the thyroid stops producing thyroid hormones, it is essential to start levothyroxine (a synthetic form
of T4 hormone). Timing and dose of levothyroxine would be decided by your referring doctor (based on
their judgment of your symptoms and blood tests). As destruction of thyroid is an irreversible process,
most patients would continue to need thyroxine replacement for their lifetime. It must not be stopped
even if blood tests of thyroid function turn out to be normal. Stopping or alteration in dose of
levothyroxine must be based only on advice of a medical expert.
- In some instances (about 10-15% cases), especially with bulky thyroid gland, long-standing Graves'
disease, those with markedly elevated anti-thyroid antibody levels, and those with normal or modestly
elevated thyroid uptake (on radionuclide thyroid scan), there is increased risk of incomplete
destruction of the thyroid gland. In such cases, RAI therapy might have to be repeated. In the interim,
ATDs might also have to be resumed.
- Preserve the summary of RAI therapy issued to you as it could be needed for future reference. Always
mention RAI therapy as part of medical history when seeing a doctor for even seemingly unrelated
illnesses. [Although destruction of thyroid is a permanent process, RAI therapy does not leave any
visible scars, so it is not possible for your doctor to make out your having undergone it unless
expressedly stated by you.]
Radiation safety aspects
- Maintain at least 1 metre distance from pregnant ladies and young children below 10 years' age (10
days).
- Avoid sharing bed with adults (6 days).
- Avoid sharing bed with pregnant ladies and young children (18 days).
- 10 days: have at least 3 litres water everyday. Do not hold urine, and try to pass urine as many times
as possible.
- Flush twice after using the washroom (10 days).
- Avoid eating from the same plate and spoons as others without washing (10 days).
- Clothes and utensils need not be washed separately.
- Patient must avoid cooking food for others (10 days).
- There is no need to use discardable plates and spoons.
- Female patients must avoid pregnancy (at least 6 months).
- Male patients must avoid fathering a child (at least 3 months).
Discomfort / risks
The following can happen within a few days, and some of the effects could last for several months.
- Pain in neck region. This can happen as the thyroid tissue gets destroyed. Pain can start within
a day and could last for 4 - 5 days. The pain is usually mild and subsides by itself. If the pain is
excessive, please consult your referring doctor or family doctor.
- Increase of thyrotoxic symptoms. Its likelihood is low, but can happen in few patients with very
large thyroid glands or uncontrolled thyrotoxicosis to begin with. Your referring consultant would have
anticipated its occurrence, and propranolol (Inderal® or Ciplar®) prescribed by your doctor helps
minimize its severity. In case the symptoms are particularly severe, please consult your referring
doctor or an endocrinologist.
- Thyroid stops functioning. It is an irreversible process, but is the intended effect of RAI
therapy.
- Reduced saliva production / dry mouth can theoretically occur, but is very unlikely at the administered
doses.
- Symptoms related to enlarged eye (Graves' orbitopathy) can worsen for several months following RAI
therapy. If your referring doctor fears its occurrence, they would have likely prescribed steroids to be
started before RAI therapy and to be continued for few days following it. Please consult your
endocrinologist and/or an ophthalmologist for further guidance.
Time taken for summary generation
Summary of RAI therapy is issued within 1 - 2 hours of last radiation dose exposure rate reading.
Preservation of this summary is strongly recommended.