Diagnosis of hidden central hypothyroidism in survivors of childhood cancer

To determine how often central hypothyroidism remains undetected by routine out-patient tests of thyroid hormone, we studied 208 pediatric cancer survivors referred for evaluation because of signs of subtle hypothyroidism or hypopituitarism. Of the 208 (68 females and 140 males), 110 had brain tumors, 14 had other head/neck tumors, 11 had solid tumors remote from head and neck, and 73 had leukemia. Patients were evaluated 1-16 yr (mean, 6.1 ± 4.1 yr) after tumor diagnosis. The nocturnal TSH surge and response to TRH were measured. Of 160 patients with free T₄ in lowest third of normal, 34% had central hypothyroidism (blunted TSH surge or low/delayed TSH peak or delayed TSH decline after TRH); 9% had central hypothyroidism with mild TSH elevation (mixed hypothyroidism). Another 16% had mild primary hypothyroidism (TSH, 5-15 mU/L). Of 48 with free T₄ in the upper two thirds of normal, 14% had central hypothyroidism; 17% had mild primary hypothyroidism. Incidence of central, mixed, and mild primary hypothyroidism 10 yr after tumor diagnosis was significantly related to total cranial radiation dose (P < 0.0001). Of 62 patients with central hypothyroidism, 34% had not developed GH deficiency. TSH surge identified 71%, and response to TRH identified 60% of those with central hypothyroidism. More than half of the slowly growing patients who have received cranial or craniospinal radiation for childhood cancer develop subtle hypothyroidism. In our study group, 92% of patients with central hypothyroidism and 27% with mixed hypothyroidism would have remained undiagnosed using baseline thyroid function tests alone. Both TSH surge and response to TRH must be evaluated to identify all of these patients.