For general screening of children who are in fairly good health and in whom CNS or pituitary disease is not strongly suspected, the newer THS assays are very useful in assessing the action of both endogenous and exogenous thyroid hormone. Those children in whom primary hypothyroidism is strongly suspected or who are already on thyroid hormone supplementation, the free T4 assay provides a useful adjunct to the TSH. If Grave's disease or factious hyperthyroidism is suspected, the total T3 assay is a useful adjunct to the TSH and the free T4. The screening of possible hypothyroidism as a result of hypothalamic or pituitary disease, the free T4 is relied on heavily, along with the response of TSH to TRH stimulation. Measurement of the nocturnal TSH surge may also be useful in this situation.