Growth hormone and thyroid function: is secondary thyroid failure underdiagnosed in growth hormone deficient patients?

Thyroid hormones and the GH/IGF-1 system show considerable mutual interference which may have physiological, pathophysiological and clinical importance. GH therapy of children and adults may induce a fall in serum T4, which seems to be due to an effect on the deiodination of T4 to T3. Animal studies suggest that the alterations in thyroid hormones in tissue may be much more prominent than the changes observed in serum. It is possible that the GH deficiency seen in the majority of patients with pituitary/hypothalamic disorders may mask secondary hypothyroidism in some patients by giving a relatively high serum T4. GH therapy may then unmask the hypothyroidism. In accordance with such a mechanism GH deficient children evaluated thoroughly to exclude secondary thyroid failure before GH administration do not develop thyroid insufficiency during GH substitution therapy. It is suggested that thyroid insufficiency should be considered in GH deficient patients with low normal serum T4.