From the Guidelines
To collect an iodine urine sample, you should first obtain a clean, sterile container from your healthcare provider or laboratory, and then collect all urine over a 24-hour period in a special container that may contain a preservative, as recommended by the most recent guidelines 1. The process of collecting an iodine urine sample involves several steps to ensure accuracy and reliability of the test results.
- Begin by washing your hands thoroughly, then collect a midstream urine sample by urinating a small amount into the toilet first, then catching the middle portion of your urine stream in the container.
- For a 24-hour urine collection, start the collection after emptying your bladder once (discard this first urine), then collect all subsequent urine for exactly 24 hours, including the first morning urine of the following day.
- During collection, keep the container refrigerated or in a cool place to prevent bacterial growth and preserve the sample.
- Avoid consuming iodine-rich foods like seafood, dairy products, and iodized salt for 24 hours before and during collection, as these can affect test results, as noted in the guidelines 1.
- Certain medications and supplements containing iodine should also be avoided, but consult your healthcare provider before stopping any prescribed medications. The importance of accurate collection is highlighted by the fact that iodine levels help evaluate thyroid function and nutritional status, as iodine is crucial for thyroid hormone production, as emphasized in the recent study 1. It is also worth noting that the 24-hour urine collection is the recommended method for assessing iodine status, as it provides a more accurate measurement of iodine excretion, as stated in the guidelines 1.
From the Research
Collecting Iodine Urine Sample
To collect an iodine urine sample, there are two main methods:
- Collecting urine over a 24-hour period
- Collecting a spot urinary sample 2 The choice of method depends on the purpose of the collection and the population being studied.
Methods for Urine Collection
- 24-hour urine collection: This method is often considered the "reference standard" for giving a precise estimate of individual iodine excretion and thereby iodine intake 2. However, it can be difficult to perform for large numbers of people.
- Spot urine sample: This method is preferable to 24-hour urinary collections in population studies, especially when collecting from a large number of people 2. The median value of spot urinary iodine concentration is a reliable measure of iodine intake in the population if the sample size is at least 500 subjects.
Considerations for Spot Urine Samples
- Timing of collection: The timing of spot urine samples can affect the results, especially if iodine excretion is high 3.
- Creatinine adjustment: Expressing urinary iodide concentration as a function of urinary creatinine can help correct for the influence of fluid intake 2.
- Number of samples needed: To reliably estimate individual iodine status, 10 repeat collections of spot urine samples or 24-hour samples may be needed 4.
Analyzing Urine Samples
- Iodine measurement: Iodine in urine can be measured using various methods, including the Sandell-Kolthoff reaction 5, 3.
- Creatinine measurement: Creatinine in urine can be measured using methods such as the Jaffe method 3.
Limitations and Challenges
- Intra-individual variation: There can be significant variation in iodine excretion within an individual over time, which can affect the accuracy of a single urine sample 4.
- Population characteristics: The accuracy of urine samples can be affected by population characteristics, such as ethnicity and diet 3.