Timed saliva sampling as a new reliable marker for assessing iodine intake

Iodine is an essential component of thyroid hormones. Therefore, sufficient iodine intake is important for adequate thyroid function. Measuring the urinary iodine concentration (UIC) is the recommended method for assessing population iodine intake, but it has certain drawbacks. In addition to urine, iodine is also excreted into saliva. Therefore, the aim of our work was to determine whether salivary iodine concentration (SLIC) measurement could replace UIC measurements, even a gold standard, urinary iodine excretion (UIE) in the 24-hour urine. We also evaluated the influence of saliva collection time on SLIC measurements.

We invited adult volunteers who first provided 24-hour urine and saliva samples collected on the same day at home, strictly according to the protocol. UIC and SLIC before and after meals were measured using the Sandell-Kolthoff method, comparable to inductively coupled plasma mass spectrometry.

The study included 104 healthy participants from whom all biological samples were completely collected. Importantly, none of them was taking thyroid medication or iodine supplements and all were without thyroid disease. A very good correlation of SLIC values at 60 and 120 min after the first and second meal and 30 min after the second, third, and fourth meals with UIE was observed (P>0.05).

We found an excellent correlation between SLIC values and UIE, where the SLIC value depends on the sampling timing and/or number of meals. SLIC measurement could replace UIE measurement and represents an important and possible new biomarker for assessing iodine intake.