Overview
Iodine is an essential trace element required for normal human growth, development, and metabolism. It is a nonmetallic chemical element and the heaviest nonradioactive halogen. In its pure form, iodine appears as a dark crystalline solid that can easily sublime into a violet-colored vapor.
In nature, iodine is never found in a free state and occurs mainly in brines and seaweeds. In human nutrition, iodine is primarily obtained through diet and fortified foods, especially iodized salt. Iodine is a critical component of the thyroid hormones thyroxine (T4) and triiodothyronine (T3), which regulate metabolic processes throughout the body.
Biological Functions of Iodine
The primary function of iodine is to support the synthesis of thyroid hormones. These hormones play a central role in controlling the body’s metabolic rate, energy production, and oxygen consumption.
Iodine is also essential for normal growth and development, particularly during pregnancy, infancy, and early childhood. Adequate iodine intake is crucial for proper brain and bone development in the developing fetus and newborn. Deficiency during these stages can result in irreversible developmental abnormalities.
Sources of Iodine
Dietary intake is the main source of iodine for humans. Iodized salt is the most common and effective source used worldwide to prevent iodine deficiency disorders. Seafood, seaweed, and marine fish are naturally rich sources of iodine.
Environmental exposure contributes to iodine intake through natural sources present in soil and water, although the iodine content of these sources varies by geographic region. Dietary supplements containing iodine are also available and are used in individuals with increased requirements or documented deficiency.
Indications for Iodine Testing
Iodine testing is used to evaluate how effectively iodine is cleared from the bloodstream by the kidneys. It plays an important role in assessing thyroid function in both hypothyroidism and hyperthyroidism.
The test is indicated in cases of suspected iodine deficiency or iodine toxicity. It is useful in patients with unexplained thyroid dysfunction and is commonly applied in research settings to evaluate iodine sufficiency or environmental exposure. Iodine testing is also valuable for monitoring iodine status during pregnancy and in neonates, as well as in population-based nutritional studies.
Assay Methods
Several analytical techniques are used to measure iodine levels. Inductively Coupled Plasma Mass Spectrometry (ICP-MS) is considered a highly sensitive and specific method for iodine estimation.
Other methods include High Performance Liquid Chromatography (HPLC), radioactive iodine assays, iodine-selective electrode assays, anodic stripping voltammetry (ASV), and colorimetric techniques. The choice of method depends on laboratory infrastructure, required sensitivity, and the type of sample being analyzed.
Methods of Sample Collection
Iodine can be measured in blood or urine samples. For serum iodine estimation, 3.0 mL of blood is collected in either an EDTA (lavender-capped) tube or a plain (red-capped) tube. Serum or plasma should be separated promptly and transferred into an acid-washed vial or a trace-element-free container to prevent contamination.
Urinary iodine estimation is commonly performed using a 24-hour urine sample, which reflects recent dietary iodine intake. No chemical preservative is required if the urine sample is refrigerated during the collection period. Urinary iodine assessment is considered more reliable for evaluating recent iodine intake, whereas serum iodine reflects current circulating iodine status. Patients should avoid iodine-containing medications or contrast agents before testing.
Reference Ranges
Normal serum iodine levels generally range from approximately 40 to 92 µg/L, although values may vary between laboratories. Serum iodine levels between 80 and 250 µg/L are commonly seen in hyperthyroidism.
Levels above 250 µg/L indicate iodine overload. For urinary iodine, adult values between 100 and 199 µg/L reflect adequate iodine intake. In pregnancy, urinary iodine levels between 150 and 249 µg/L are considered adequate to support fetal development.
Interpretation of Results
Low serum iodine levels suggest iodine deficiency and warrant evaluation of thyroid function. Iodine deficiency increases the risk of goiter, hypothyroidism, developmental brain damage in infants and children, and increased neonatal mortality.
High serum iodine levels indicate iodine excess or toxicity. Excess iodine intake may lead to thyroid dysfunction and can cause iodine-induced hyperthyroidism, also known as the Jod–Basedow effect. High iodine levels may also trigger autoimmune thyroiditis or hypothyroidism in susceptible individuals.
Clinical Utility
Iodine testing is used to diagnose nutritional iodine status at both individual and population levels. It supports public health programs aimed at monitoring iodized salt coverage and preventing iodine deficiency disorders.
The test is useful for monitoring iodine supplementation and for assessing exposure to iodine-containing medications such as amiodarone or iodinated contrast agents used in imaging studies.
Limitations
Serum iodine levels are influenced by daily dietary intake and kidney function, leading to short-term fluctuations. Iodine testing is not recommended as a routine screening test and is mainly used for specific clinical indications or research purposes.
Because of variability in serum iodine levels, 24-hour urinary iodine estimation is considered the preferred method for assessing overall iodine status.
