Is there compensated hypothyroidism in infancy?

Abstract

The state-mandated newborn thyroid screening program may uncover infants who exhibit normal thyroxine (T4) levels with various degrees of hyperthyrotropinemia. To elucidate further the thyroid status, the basal metabolic rate (BMR) of 10 infants (7 boys, 3 girls; aged 9 to 63 days) was studied by indirect calorimetry. They were clinically euthyroid and healthy with no evidence of overt biochemical hypothyroidism (low T4, high thyroid-stimulating hormone [TSH]). Confirmatory testing indicated that all infants had normal serum T4 levels for age (mean +/- SD: 10.3 +/- 3.2 micrograms/dL). However, serum TSH levels varied from 2.3 to 99.2 microU/mL. In 4 infants (2 boys, 2 girls) the BMR was low (38.1 +/- 4.1 kcal/kg per day), while the other 6 patients (5 boys, 1 girl) demonstrated BMRs within the normal range (49.6 +/- 1.9 kcal/kg per day, P less than .001). The serum TSH levels were above 7.0 microU/mL among those infants with a low BMR, whereas the serum TSH levels were always below 6.0 microU/mL among the normometabolic infants. All infants who had a low BMR received thyroid therapy and promptly became normometabolic (BMR: 48.7 +/- 1.0 kcal/kg per day) with suppression of TSH levels (3.2 +/- 1.3 microU/mL) within 3 weeks of therapy, while their serum T4 levels remained within the normal range. The observed normalization of BMR in parallel to reduction of TSH levels following thyroid replacement therapy strongly suggests that these patients demonstrated a hypometabolic state, despite normal serum T4 levels. Therefore, the assessment of BMR may help define subclinical hypothyroidism in infancy in conjunction with a close monitoring of TSH concentration.