Urinary Iodine Clearance (UIC)

The Urinary Iodine Clearance (UIC) test is the most commonly used lab analysis to assess iodine status. During pregnancy, this lab changes based on increased iodine clearance and may not be as accurate in determining iodine function. In this article, I'll cover the UIC test, how pregnancy physiology changes this test, and common reasons for discrepancies. 

UIC Testing

The urinary iodine clearance (UIC) test measures the amount of iodine present in a urine sample. It can be done using a random sample, a 24-hour sample, or an iodine:creatinine ratio.

The Random UIC is the most common assessment for iodine deficiency used in research.
The 24-hour UIC is slightly more accurate than the Random UIC, but it is rarely assessed in research and can be variable based on consumption and methodological errors. There are no established pregnancy ranges.

The iodine:creatinine ratio is corrected for urinary output and kidney function, adding accuracy to the UIC spot test.

In cases of iodine deficiency, more iodine is reabsorbed, and less is excreted. When iodine levels are sufficient, more iodine is excreted through the urine.

Approximately 38% of Americans are classified as iodine deficient, with a
UIC <100 μg/L.

Iodine, Random Urine

Iodine, 24-Hour Urine

Iodine: Creatinine Ratio

UIC in Pregnancy

Urinary iodine clearance increases during pregnancy, with the highest clearance seen during the first trimester.

High UIC in Pregnancy

Levels <250 μg/L are protective against hyperthyroidism. There is a bell curve in iodine and thyroid function. Excess iodine limits iodine uptake to the thyroid.

Most Common Causes of High UIC in Pregnancy

• Dehydration
• Excess Supplementation
• Certain Medications (Diuretics, Progesterone)
• Hyperthyroidism
• Vitamin A Deficiency

Low UIC in Pregnancy

​There is limited data on the consequences of low UIC in pregnancy. (See Serum Iodine Article)

Most Common Causes of Low UIC in Pregnancy

• Iodine Deficiency 
• Kidney Disease
• Hypothyroidism
• Overhydration

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Medical Disclaimer:
This content is for educational purposes only and is intended for licensed medical professionals. It is based on current research, subject to change, and not intended to diagnose, treat, or prevent any medical condition. This information does not replace consultation with a qualified healthcare provider. Seek professional medical evaluation and treatment from a licensed provider. Use of this information is at your own risk.​

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