Objective To assess iodine status in four separate groups — pregnant women, postpartum women, patients with diabetes mellitus and volunteers. Design and setting Prospective cross‐sectional study at a tertiary referral hospital in Sydney. Participants 81 pregnant women attending a “high risk” obstetric clinic; 26 of these same women who attended three months postpartum; 135 consecutive patients with diabetes mellitus attending the diabetes clinic for an annual complications screen; and 19 volunteers. There were no exclusion criteria. Methods Spot urine samples were obtained, and urinary iodine was measured by inductively coupled plasma mass spectrometer. Outcome measures Iodine status based on urinary iodine concentration categorised as normal (>100 μg/L), mild deficiency (51–100 μg/L) and moderate to severe deficiency (<50 μg/L). Results Moderate to severe iodine deficiency was found in 16 pregnant women (19.8%), five postpartum women (19.2%), 46 patients with diabetes (34.1%) and five volunteers (26.3%). Mild iodine deficiency was found in an additional 24 pregnant women (29.6%), nine postpartum women (34.6%), 51 patients with diabetes (37.8%) and 9 normal volunteers (47.4%). Median urinary iodine concentration was 104 μg/L in pregnant women, 79 μg/L in postpartum women, 65 μg/L in patients with diabetes mellitus and 64 μg/L in volunteers. Conclusions The high frequency of iodine deficiency found in our participants suggests that dietary sources of iodine in this country may no longer be sufficient. Further population studies are required.