Table 1.

Clinical situations in which measurements of serum TSH alone may yield misleading results.

Condition	Serum TSH	Consequences of Clinical Action based on Serum TSH value Alone	Serum FT4
Heterophile antibodies	Normal	Failure to diagnose thyrotoxicosis	High
Central hypothyroidism	Normal*	Failure to diagnose hypothyroidism and investigate hypothalamic-pituitary structure function	Low
TSH-secreting pituitary adenoma	Normal*	Failure to diagnose thyrotoxicosis and investigate pituitary structure and function	High
Thyroid hormone resistance	Normal*	Failure to recognize the condition	High
Poor compliance with T4 therapy	High	Inappropriate increase in dose of T4	High
Delayed recovery of TSH secretion after treatment of hyperthyroidism	Normal or low	Failure to diagnose impending hypothyroidism	Low

*Serum TSH concentrations may also be high in these conditions, which should prompt measurements of serum FT4 and further investigation.