Growth and thyroid function in children treated with growth hormone

We examined the effect of growth hormone (GH) therapy on thyroid function in 57 children with isolated GH deficiency and whether this effect could influence their growth response. Thyroid function and insulin-like growth factor I levels were measured before and after 3, 6, and 12 months of recombinant-GH therapy (20 U/m 2 per week, given subcutaneously), after a 1-month withdrawal from therapy, and after a further 6 months of GH administration. The serum concentration of triiodothyronine (T 3) and the T 3/T 4 (thyroxine) ratio increased after 12 months of GH treatment, whereas total T 4 and free T 4 levels decreased; thyrotropin levels did not change significantly during treatment but increased after a 1-month withdrawal. After a further 6 months of GH therapy, an increase in T 3 levels and in the T 3/T 4 ratio and a decrease in total T 4 and free T 4 levels were found again, and thyrotropin levels decreased. The increment in growth velocity after 12 months of therapy correlated positively with the T 3/T 4 ratio and negatively with total T 4 and free T 4 values. These data confirm in children a GH-induced enhancement of peripheral conversion of T 4 to T 3. This effect appears to be more evident in children who are most sensitive to GH in terms of growth-promoting activity.