Background: The authors previously reported on the development of thyroid d
ysfunction and autoimmunity during 1-year treatment of patients with MS wit
h interferon-beta 1b (IFN beta-1b). Objective: To evaluate the evolution of
incident thyroid disease and the possible development of more thyroid dise
ase during longer term therapy. Patients: The authors studied 31 patients (
aged 34 +/- 7 years; 21 women) with relapsing-remitting MS during 3 years o
f IFN beta-1b treatment. Systematic thyroid assessment was performed every
3 or 6 months, depending on the development of thyroid disease. Results: Af
ter the first year of IFN beta-1b treatment, no further cases of thyroid di
sease were observed. Among the six patients with early incident subclinical
hypothyroidism, thyroid dysfunction persisted only in those with baseline
autoimmune thyroiditis (n = 2). The three patients who developed transient
hyperthyroidism remained euthyroid throughout the treatment course. A posit
ive autoantibody titer was continually detected in only two out of five pat
ients without baseline autoimmunity. Conclusions: The risk of thyroid disea
se seems related to IFN beta-1b treatment during the first year only, parti
cularly in patients with preexisting thyroiditis. Furthermore, incident thy
roid dysfunction is generally transient and mild in degree. Indeed, we reco
mmend a routine systematic thyroid assessment only in patients with baselin
e thyroiditis. During the first year of therapy, serum thyroid-stimulating
hormone measurement should suffice as first line test; a systematic thyroid
assessment is only useful for those patients with incidental and persisten
t dysfunction. Further studies with many patients will be necessary to conf
irm our suggestions as broad clinical guidelines.