**Background:** Iodine deficiency is a leading cause of preventable brain damage, particularly during the first 1000 days of life. Poland introduced mandatory iodisation of household salt in 1997 (30 ± 10 mg KI/kg salt) after voluntary programmes failed. This study aimed to evaluate the current iodine status of vulnerable populations (school children, pregnant women, lactating women) in Poland in 2017, as part of the National Health Programme.
**Methods:** The study was conducted between October and December 2017 in five randomly selected regions of Poland (central, north, north-east, south, south-east). Participants included 300 pregnant women (median age 30 years, median 29 weeks gestation), 100 lactating women (median age 31 years, median 7 weeks lactation), and 1000 school children aged 6–12 years (517 girls, 483 boys). Urinary iodine concentration (UIC) was measured in spot morning urine samples using the Sandell–Kolthoff reaction. Serum thyroglobulin was measured in pregnant and lactating women using electrochemiluminescence (Roche Eclesys Tg II assay). Iodine status was interpreted according to WHO/UNICEF criteria: adequate iodine intake defined as median UIC 150–249 µg/l for pregnant women, ≥100 µg/l for lactating women, and 100–299 µg/l for school children; additionally, <20% of school children should have UIC <50 µg/l. For thyroglobulin, iodine sufficiency was defined as median <13 ng/ml and <3% of subjects with values >40 ng/ml (based on school children reference). Statistical analysis used medians, interquartile ranges (IQR), 95% bootstrapped CIs, and Kruskal–Wallis ANOVA.
**Key Results:**
- **Pregnant women:** Median UIC was 111.6 µg/l (IQR 134.3, 95% BCI 97.8–124.2). No significant inter-regional difference (P = 0.3388). Median serum thyroglobulin was 13.3 ng/ml (IQR 17.0); 8% (24/300) had thyroglobulin >40 ng/ml, and 7.3% had >44 ng/ml. According to WHO criteria, the sample was iodine-deficient.
- **Lactating women:** Median UIC was 68.0 µg/l (IQR 74.7, 95% BCI 50.9–74.9). Significant inter-regional difference (P = 0.0143). Median serum thyroglobulin was 18.5 ng/ml (IQR 20.7); 18% (18/100) had thyroglobulin >40 ng/ml. According to WHO/IGN criteria, the sample was iodine-deficient.
- **School children:** Median UIC was 119.8 µg/l (IQR 92.4). Significant inter-regional variation (P = 0.0000), with highest values in south and south-east regions. 94 children (9.4%) had UIC <50 µg/l; only the northern region exceeded the 20% threshold (23.7%). According to WHO criteria, the sample was iodine-sufficient.
**Clinical Implications:** The study demonstrates that Poland's mandatory household salt iodisation has achieved sustainable iodine sufficiency in school children (representing the general population), but fails to meet the increased iodine requirements during pregnancy and lactation. The median UIC in pregnant women (111.6 µg/l) is well below the recommended 150 µg/l, and lactating women (68.0 µg/l) are even further from the ≥100 µg/l target. The elevated thyroglobulin levels (8% of pregnant and 18% of lactating women >40 ng/ml) corroborate inadequate iodine status. These findings align with global data showing that even in countries with successful salt iodisation programmes, pregnant women often remain iodine-deficient. The authors recommend expanding iodisation to include salt used in the food industry (universal salt iodisation) and diversifying dietary iodine sources (e.g., iodised bread, as in Australia). They also stress the need for regular monitoring and potential adjustment of iodine content in salt to prevent both deficiency and excess. Limitations include subnational representation (western voivodships not included), volunteer recruitment bias, small sample size for lactating women, and lack of correction for anti-thyroglobulin antibodies.