An abnormality of the hypothalamic-pituitary-thyroid axis may be present in chronic illness, CNS injury, growth hormone deficiency, and idiopathic short stature. This may be evident in a low nocturnal TSH surge. The TSH surge will be tested in children with these conditions. Patients with a low TSH surge and matched patients with a normal surge will be treated with levothyroxine for 6 months and their growth and clinical responses compared.