Wallach's Interpretation of Diagnostic Tests: Pathways to Arriving at a Clinical Diagnosis (1241 page)

   Because of widespread dietary use of iodine in the United States, RAIU should not be used to evaluate euthyroid state.

   Increased by withdrawal rebound (thyroid hormones, propylthiouracil), increased iodine excretion (e.g., diuretics, nephrotic syndrome, chronic diarrhea), decreased iodine intake (salt restriction, iodine deficiency).

THYROID-STIMULATING HORMONE (TSH)

   Definition

   This glycoprotein hormone of 28–30 kDa is composed of alpha and beta subunits. It is secreted by the anterior pituitary. TSH controls the biosynthesis and release of thyroid hormones T
₄
and T
₃
.

   
Normal range:
0.5–6.3 μIU/mL, depending on age and sex (Table 16.78).

TABLE 16–78. Normal Ranges of TSH According to Age and Sex

   Use

   Sensitive measure of thyroid function. First line of test for suspected thyroid disorders

   Assessing true metabolic status

   Screening for euthyroidism

   Normal level in stable ambulatory patient not on interfering drugs excludes thyroid hormone excess or deficiency.

   TSH is recommended as the initial test rather than T
₄
.

   Screening is not recommended for asymptomatic persons without suspicion of thyroid disease or for hospital patients with acute medical or psychiatric illness.