Iodine Excretion, Urine 24 Hours

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Subject: Iodine Excretion, Urine 24 Hours

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Definition

  • Iodine is an essential component of T4 and T3, and it must be provided in the diet. Inadequate iodine intake leads to inadequate thyroid hormone production, and all the consequences of iodine deficiency stem from the associated hypothyroidism. However, iodide excess can also cause thyroid dysfunction. Goiter is the most obvious manifestation of iodine deficiency. Low iodine intake leads to reduced T4 and T3 production, which results in increased thyrotropin (TSH) secretion in an attempt to restore T4 and T3 production to normal.

  • Normal range:

    • International groups recommend the following median urinary iodine concentration as the best single indicator of iodine nutrition in populations:

      • Severe deficiency: 0–0.15 μmol/L (0–19 μg/L)

      • Moderate deficiency: 0.16–0.38 μmol/L (20–49 μg/L)

      • Mild deficiency: 0.40–0.78 μmol/L (50–99 μg/L)

      • Optimal iodine nutrition: 0.79–1.56 μmol/L (100–199 μg/L)

      • More than adequate iodine intake: 1.57–2.36 μmol/L (200–299 μg/L)

      • Excessive iodine intake: 2.37 μmol/L (300 μg/L)

    • The range in which the median falls is more important than the precise number.

Use

  • Diagnosis of transient thyroid dysfunction and iodine-induced hyperthyrosis

  • Biochemical indicator for the assessment of iodine status

  • Monitoring iodine excretion rate as an index of daily iodine replacement therapy

  • Correlating total body iodine load with 131I uptake studies in assessing thyroid function

Interpretation

Increased In

  • Dietary excess

  • Recent drug or contrast media exposure

Decreased In

  • Dietary deficiency

Limitations

  • Urinary iodine levels are influenced by gender, age, sociocultural and dietary factors, drug interferences, geographic location, and season.

  • In most instances, it provides little useful information on long-term iodine status of the individual, since the results obtained merely reflect the dietary iodine intake.

  • Administration of iodine-based contrast media and drugs containing iodine, such as amiodarone, will yield elevated results.

  • High concentrations of gadolinium are known to interfere with most metals tests. If gadolinium-containing contrast media has been administered, a specimen should not be collected for 48 hours.

  • Frozen specimens sometimes result in falsely lowered results.

Suggested Reading

International Council for the Control of Iodine Deficiency Disorders. WHO. UNICEF. Assessment of Iodine Deficiency Disorders and Monitoring their Elimination. A Guide for Programme Managers, 2nd ed. Geneva, Switzerland: World Health Organization; 2001.
 
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