Table 1.

Biomarkers of population iodine status in infants, toddlers, and lactating women

Biomarker	Population group	Specimen	Analytical method	Advantages	Disadvantages	Threshold defining iodine sufficiency in populations
UIC	Infants/toddlers lactating women	Spot urine   Infant samples can be collected using urine pads or urine bags	Spectrophotometric (Sandell-Kolthoff reaction)   ICP-MS	Noninvasive   Reflects recent iodine intake (within h)   Assess intake from all dietary sources   External quality control program in place (307)	High intraindividual and interindividual variability due to large variation in iodine intake and urine volume (249, 306)   Large sample size needed(306)   Not useful for individual assessment unless ≥ 10 repeated samples collected (306)   UIC in lactating women should be assessed along with BMIC as fractional iodine excretion may vary in urine and breast milk (250)	Median UIC > 100 µg/L in lactating women and infants recommended by WHO (176), but evidence for this threshold in lactating women, infants, and toddlers is weak. Median UIC >200 μg/L likely more appropriate in infants (see Fig. 3)   Criteria indicating deficient, optimal, and excessive iodine intake should be defined
BMIC	Lactating women	Spot breast milk	ICP-MS	Noninvasive   Reflects recent iodine intake (within h)	High intraindividual variability due to large day-to-day variability in iodine intake	Not yet adopted (21)
					Not reliable for individual assessment	Observational studies suggest median BMIC of between 100 and 200 µg/L indicate iodine sufficiency(18, 250) (Fig. 3)   Criteria indicating optimal iodine nutrition to be defined   Assay specific reference ranges: to be defined for most assays (physiological decline during first mo of life must be considered)
Tg	Infants/toddlers, lactating women	Serum or DBS	ELISA	Venopuncture	Values elevated at deficient and excessive iodine intakes and should be accompanied by UIC   Wide interassay variation   Affected by degradation when stored under hot and humid conditions (DBS)
				Simple collection by finger or heel prick and storage on filter paper
				Small sample volume
				Reflects intermediate iodine status (wk to mo)
Neonatal TSH	Neonates 2-5 d after birth	Serum or DBS	Various immunoassays	Collection by heel prick and storage on DBS is simple	Should be taken at least 48 h after birth to avoid TSH surge	Prevalence < 3% of values > 5 mIU/L indicates iodine sufficiency (176), but threshold < 5 mIU/L may be more sensitive to detect mild iodine deficiency
				International reference range available	Primarily reflects exposure to iodine deficiency during pregnancy   Reflects population risk of moderate-to-severe iodine, but insensitive to mild iodine deficiency   May be confounded by use of iodine-containing antiseptics at birth
				Measures thyroid function at a particularly susceptible age

Abbreviations: BMIC, breast milk iodine concentration; DBS, dried blood spots; ELISA, enzyme-linked immunosorbent assay; ICP-MS, inductively coupled plasma mass spectrometry; Tg, thyroglobulin; TSH, thyrotropin; UIC, urinary iodine concentration.