Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1244 page)

   Secondary pituitary or hypothalamic hypothyroidism (e.g., tumor, infiltrates)

   Euthyroid sick patients (some patients)

   Acute psychiatric illness

   Severe dehydration

   Drug effect, especially large doses (use FT
₄
for evaluation)

   Glucocorticoids, dopamine, dopamine agonists (bromocriptine), levodopa, T
₄
replacement therapy, apomorphine, and pyridoxine; normal or low T
₄

   Antithyroid drug for thyrotoxicosis, especially early in treatment; normal or low T
₄

   Assay interference (e.g., antibodies to mouse IgG, autoimmune disease)

   First trimester of pregnancy

   Isolated deficiency (very rare)

   Limitations

   TSH may be normal in

   Central hypothyroidism: In the absence of hypothalamic or pituitary disease, normal TSH excludes primary hypothyroidism.