Up to 20% of patients, usually those with a large thyroid remnant, given sufficient 131I to deliver about 500 Gy (50000 rads) develop thyroiditis , which occurs less often when 1110 MBq (30mCi) of 131I is administered . Within a week, painful swallowing, neck and ear pain, thyroid tenderness and swelling, and transient mild thyrotoxicosis may occur, which is usually transient, requiring no therapy.
Up to one-third of patients develop acute and/or chronic parotid or submandibular gland sialadenitis after 131I therapy . Symptoms may occur within 24 hours and are more likely when large amounts of 131I have been given to a patient with a small thyroid remnant . Chewing gum, sucking on lemon candies, 24 hours after 131I administration, and hydration may prevent the sialadenitis and xerostomia. Still, most patients experience intermittent painless salivary gland swelling that is caused by an epithelial plug in the salivary duct, beginning several months after 131I therapy and lasting a few hours, reminiscent of a salivary duct stone. There is a salty taste when the salivary pressure is reduced spontaneously or by manual pressure on the parotid. Often misdiagnosed as infectious parotitis, it requires no therapy and usually improves spontaneously within about a year; however, it may be associated with a chronic xerostomia . Most patients have reduced taste for several weeks, sometimes with transient tongue pain .
A recent study found that conjunctivitis and nasolacrimal drainage system obstruction occurred 3 to 16 months after administering larger amounts of 131I (6660 ± 555MBq, 180 ± 15mCi) . This occurred in 3.4% of 563 patients in one study  but its incidence may be higher since these patients were not systematically evaluated nor questioned about tearing. The reported patients, who were likely to be the worst cases, often required surgical therapy .
About two-thirds of patients given 7400 MBq (200mCi) or more develop mild radiation sickness with headache, nausea and occasional vomiting that begins about 4 hours after 131I administration and resolves in about 24 hours. It rarely occurs with less 131I .
This is the most serious acute complication, which may result from 131I therapy or TSH stimulation, induced either by thyroid hormone withdrawal or rhTSH stimulation . Serious symptoms can occur rapidly when the tumor is in a critical location such as the brain, spinal cord, or airway [147,148]. Likewise, pain may occur in bone metastases. Pretreatment with corticosteroids may minimize these problems, but surgery for spinal lesions and isolated brain metastases should be considered before TSH stimulation or 131I therapy . Vocal cord paralysis is reported in patients with a large amount of functioning thyroid tissue in close proximity to the vocal cords . Transient peripheral facial nerve palsy also occurs rarely after high dose 131I therapy .
A slight reduction in platelets and white blood cell counts may follow 131I therapy but is typically transient and asymptomatic . Pancytopenia can follow very large doses of 131I, which may require transfusions, but this is usually reversible [151,152].
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