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Diagnosis of thyroid dysfunction is confirmed by abnormalities in thyroid hormone levels in plasma. With the availability of ultrasensitive assay of TSH, it can be used to identify both primary hypothyroidism and hyperthyroidism (with TSH levels being high and low respectively). Overdose of thyorxine replacement can also be identified, along with diagnosis of subclinical hypothroidism, and evaluation of thyroid function in hospitalized patients, and in pregnancy. Subclinical hypothyroidism was diagnosed by an exaggerated TSH response to intravenous TRH bolus. Prenatal diagnosis and treatment of hypothyroidism is also possible.