Natural history of thyroid function tests over 5 years in a large pediatric cohort

Context: Because clinical manifestations of thyroid disorders are variable and subtle in children and adolescents, thyroid function tests are often repeated in patients with nonspecific symptoms. Objectives: The objective of the study was to determine the natural history of initial abnormal TSH and define populations at greater risk for developing a subsequent thyroid dysfunction. Methods: A total of 121,052 of 1.043 million outpatients aged 0.5-16 yr insured by the Clalit Health Medical Organization had a TSH determination in 2002 and follow-up to 2007. Extracted from the Clalit Health Medical Organization database were their demographic data, referral diagnoses, and laboratory results (TSH, free T₄, thyroid antibodies). Excluded were patients with overt hypothyroidism or hyperthyroidism on initial testing. Results: Results of 96.5% of initial serum TSH concentrations were normal (0.35-5.5 mIU/liter),0.2% were low (<0.35 mIU/liter), 2.9% elevated (>5.5 to ≤10 mIU/liter), and 0.4% highly elevated (>10 mIU/liter). The frequency of TSH testing increased with age and female gender. During follow-up, repeated (two to more than four) TSH tests were performed in 45.7% of the patients. In the second TSH determination, normal TSH was documented in 40, 73.6, and 78.9% of those whose initial serum TSH was highly elevated, elevated, and low, respectively, and in 97% of those with normal initial TSH. Predictive factors for a sustained highly elevated TSH were initial TSH greater than 7.5 mIU/liter (P = 0.014) and female gender (P = 0.047). Conclusions: In the pediatric population, initial normal or slightly elevated TSH levels are likely to remain normal or spontaneously normalize without treatment. Patients with initial levels greater than 7.5 mIU/liter, particularly girls, are at a greater risk for sustained abnormal TSH levels.