To describe the response of thyroid-stimulating hormone (TSH) to thyroid-releasing hormone in children and adolescents with Prader-Willi syndrome (PWS), and to compare TSH and total thyroxine (TT4) concentrations measured on neonatal screening for congenital hypothyroidism in children with PWS and controls.

All participants had genetically confirmed PWS. The TSH responses to thyroid-releasing hormone, free thyroxine (fT4), and free triiodothyronine (fT3) were measured in 21 subjects (14 females and 7 males; mean age, 6.4 years). Capillary TT4 was measured on neonatal screening samples from 23 subjects with PWS (14 females and 9 males), each of whom was matched for birth weight and sex with 4 anonymized controls.

One subject with PWS had tertiary hypothyroidism. TSH level increased from 1.37 mU/L at baseline to 39.6 mU/L at 20 minutes, 47.2 mU/L at 40 minutes, 44.5 mU/L at 60 minutes, and 47.2 mU/L at 120 minutes. fT4 concentration was 6.3 pmol/L, and fT3 concentration was 4.6 pmol/L. In the other 20 subjects, mean TSH level was 1.9 mU/L (range, 0.8-4.2 mU/L) at baseline and 21.8 mU/L (range, 10.0-46.7 mU/L) at 20 minutes (peak). Mean fT4 concentration (10.4 pmol/L; range, 8.2-13.5 pmol/L) was in the lower one-third of the normal range in 18 subjects, and mean fT3 concentration (6.1 pmol/L; range, 4.8-8.4 pmol/L) was above the median in 13 subjects. In neonates, mean TSH level was 3.1 mU/L (range, 0.4-10.0 mU/L) in subjects with PWS versus 3.3 mU/L (range, 0.0-7.0 mU/L) in controls, and mean TT4 in subjects with PWS was 111% (range, 17%-203%) that of controls (P = not significant).

Thyroid function was normal in our newborn subjects. In older children, frank hypothyroidism was found in only 1 of our 21 subjects. Thus, levothyroxine treatment should not be routinely prescribed to youth with PWS.