Epidemiology

The true prevalence of painless thyroiditis is difficult to ascertain and varies by geographic location, how frequently patients are screened, and whether all thyrotoxic patients are assessed with diagnostic radionucleotide imaging.

In one series, up to 23% of all thyrotoxic patients had painless thyroiditis, while other studies have reported considerably lower percentages.[2][3][4] A crude estimate of incidence using the prevalence of painless thyroiditis among thyrotoxic patients (23%) and the incidence rate for thyrotoxicosis from population studies suggests an incidence rate for painless thyroiditis of approximately 0.28 cases per 1000 person-years.[6] The condition is twice as likely to occur in women and can occur in all age groups, although the mean age of onset is in the third decade.[2][3]

Postpartum thyroiditis occurs after 7% of pregnancies (range 1% to 17%), although only one third of these patients present with thyrotoxicosis (in some, the thyrotoxic phase goes unnoticed).[7] It may also occur after miscarriage or medical abortion. It has also been reported after miscarriage or therapeutic abortion.[5]

Thyroid dysfunction is seen in up to 40% of patients treated with immunomodulatory drugs such as interferon alfa, tyrosine kinase inhibitors (e.g., sunitinib), and monoclonal antibodies (e.g., alemtuzumab, ipilimumab, nivolumab). However, only a minority of these patients present with transient thyrotoxicosis.[8][9][10][11] Similarly, while patients taking lithium have a high prevalence of hypothyroidism, the incidence of painless thyroiditis is 1.3 per 1000 patient-years, or about 5-fold that of the general population.[6] 

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