Laboratory tests for evaluating thyroid therapy.

Appropriate dosage of levothyroxine for the treatment of hypothyroidism is assessed by determining the serum thyroxine (T4) concentration in secondary and tertiary types. In primary hypothyroidism, the optimal thyroid replacement is that which induces a normal thyroid-stimulating hormone level and a normal TSH response to administration of thyrotropin-releasing hormone. Hypothyroidism often occurs in the management of hyperthyroidism. Serial serum TSH measurements help in the early detection of hypothyroidism, whereas serum triiodothyronine (T3) aids in prompt recognition of recurrence of hyperthyroidism.