Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1242 page)

   Initial screening and diagnosis for hyperthyroidism (decreased to undetectable levels except in rare TSH-secreting pituitary adenoma) and hypothyroidism

   Especially useful in early or subclinical hypothyroidism before the patient develops clinical findings, goiter, or abnormalities of other thyroid tests

   Differentiation of primary (increased levels) from central (pituitary or hypothalamic) hypothyroidism (decreased levels)

   Monitoring of adequate thyroid hormone replacement therapy in primary hypothyroidism, although T
₄
may be mildly increased (up to 6–8 weeks before TSH becomes normal). Serum TSH suppressed to the normal level is the best monitor of dosage of thyroid hormone for treatment of hypothyroidism

   Monitoring adequate thyroid hormone therapy in the suppression of thyroid carcinoma (should suppress to <0.1 μIU/mL) or goiter or nodules (should suppress to subnormal levels) with third- or fourth-generation assays

   Replacement of TRH stimulation test in hyperthyroidism, because most patients with euthyroid TSH level have a normal TSH response and patients with undetectable TSH level almost never respond to TRH stimulation

   Interpretation

Increased In

   Primary untreated hypothyroidism. The increase is proportionate to the degree of hypofunction, varying from 3 times normal in mild cases to 100 times normal in severe myxedema. A single determination is usually sufficient to establish the diagnosis.

   Patients with hypothyroidism receiving insufficient thyroid hormone replacement therapy.

   Patients with Hashimoto thyroiditis, including those with clinical hypothyroidism and about one third of those patients who are clinically euthyroid.

   Use of various drugs: amphetamines (abuse), iodine-containing agents (e.g., iopanoic acid, ipodate, amiodarone), and dopamine antagonists (e.g., metoclopramide, domperidone, chlorpromazine, haloperidol).

   Other conditions (test is not clinically useful).