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

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.

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.

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 T(4), thyroid antibodies). Excluded were patients with overt hypothyroidism or hyperthyroidism on initial testing.

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). 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).

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.