Thyroid hormone in hypopituitarism, Graves' disease, congenital hypothyroidism, and maternal thyroid disease during pregnancy.

Although measurement of serum thyroid-stimulating hormone (TSH) is the single best test to diagnose thyroid disorders and monitor treatment, there are certain situations in which the TSH level cannot be used as a guideline. In the diagnosis of children with hypopituitary hypothyroidism, TSH is often "inappropriately" normal. Treatment is aimed at adjusting the l-thyroxine dose to maintain the serum free thyroxine (T4) in the upper half of the normal range for age. In children with Graves' disease, serum TSH can be suppressed for several months after a euthyroid state is restored, so the clinician must rely on serum free T4 and triiodothyronine (T3) levels. Up to 30% of infants and 10% of children with congenital hypothyroidism have a mildly elevated TSH level despite other indications that the thyroid hormone dosage is correct. Such resistance to thyroid hormone at the pituitary gland diminishes with age. Pregnancy is another condition in which serum TSH levels are altered (slightly lower); in this situation, it is the result of elevated human chorionic gonadotropin levels that cross-react with the TSH receptor. This alteration must be taken into account when diagnosing or treating maternal thyroid disorders during pregnancy.