Pediatric thyroid testing issues.

Thyroid problems are common in children. While serum thyroid function tests lead to an accurate diagnosis in most patients, unique patient situations can produce misleading results. Total T4 measurements can incorrectly suggest hypothyroidism in congenital thyroid binding globulin (TBG) deficiency and hyperthyroidism in TBG excess, as seen in high estrogen states. Free T4 (FT4) measurement techniques involve either physical separation of unbound thyroxine from serum binding proteins or estimation of FT4 levels in the presence of binding proteins. These estimation techniques are susceptible to under- or over-estimation of FT4 levels when binding proteins are low or high. Other complicating factors arise in the setting of prematurity or systemic non-thyroidal illness (NTI), simulating central hypothyroidism. Thyroid stimulating hormone (TSH) levels in children have a wider normal range than in adults and are affected by drugs and NTI. Additionally, heterophile and anti-T4 or anti-TSH antibodies can interfere with accurate T4 or TSH measurement.