Having previously found low levels of free thyroxine in pregnant women, 13 and in light of the adverse consequences of iodine deficiency during pregnancy, we initially set out to test the iodine status of a group of pregnant women. We subsequently included other groups to widen our investigation of iodine status.

The accepted minimum adequate level of urinary iodine is 100 µg/L, and levels above this are considered normal. 1,6-9,12 Urinary iodine concentrations below 25 µg/L are classified as severe deficiency, and are associated with an increased risk of cretinism; 26-50 µg/L is classified as moderate deficiency, and 51-100 µg/L is regarded as mild iodine deficiency. 1,6-9,12 The World Health Organization (WHO) recommends that the median urinary iodine concentration for populations as a whole should be more than 100 µg/L, that less than 20% of the population should have a urinary iodine concentration below 50 µg/L, and that no cretinism occurs. 12

The recommended daily intake (RDI) of iodine is 100 µg daily for the general population and 150-200 µg daily for women who are pregnant or breastfeeding 6-8,10 (iodine demand increases during pregnancy because of increased renal clearance and fetal iodine transfer).

One of the patients with diabetes had recently received iodine-containing intravenous contrast medium during a coronary angiogram, and one was taking amiodarone. No other participant was known to have received contrast medium, or to be taking amiodarone or iodine supplements.

One of the 81 pregnant women had thyrotoxicosis as a result of Graves' disease -- she particpated before commencing therapy. Twenty-two of the patients attending the diabetes clinic (16.1%) had type 1 diabetes, 103 (76.3%) had type 2 diabetes and 10 (7.5%) had impaired glucose tolerance.

Participants thus comprised 81 consecutive pregnant women who attended the obstetric clinic between 1 August 1998 and 1 April 1999, 26 of these same women who were reassessed at three months postpartum, 135 consecutive patients who attended the diabetes clinic for an annual complications screen between 1 November 1998 and 1 February 1999, and 19 volunteers recruited between 1 February and 1 July 1999. All participants provided a routine urine sample. There were no exclusion criteria.

We did not seek ethical approval for this study as it involved no deviation from usual care, except in the case of the 19 volunteers who agreed to provide a urine sample.

As part of routine care, 70 of the 81 pregnant women and 121 of the 135 patients with diabetes had thyroid function tests. Free thyroxine (FT 4 ) and thyroid-stimulating hormone (TSH) levels were measured by means of an automated chemiluminescence system (Chiron Diagnostics, Scoresby, Vic.).

Twenty-four-hour urinary iodine measurement may be used to assess iodine status, 6 but this method can be unreliable because of incorrect or incomplete collection, 15 and is less practical than spot samples for population surveys. 9,12 To compare this method with our spot sampling, we selected six pregnant women (on the basis of their spot urine concentrations to cover a range of values) who collected 24-hour urine samples for iodine content measurement. The correlation was highly significant ( r 2 = 0.82), thus confirming that spot urine samples were a reliable way of measuring iodine status.

Some investigators use the urinary iodine/creatinine ratio to determine iodine status. 1,14 We thus measured urinary creatinine by the Creatinine Jaffa method (Boehringer Mannheim Systems, Mannheim, Germany) and calculated iodine/creatinine ratios (µg iodine/g creatinine) for each participant. The correlation between urinary iodine and iodine/creatinine ratio was high for non-pregnant participants ( r = 0.969; P < 0.001) and lower for the pregnant group ( r = 0.419; P < 0.001).

Box 5 shows TSH levels and FT 4 levels versus iodine status in pregnant and non-pregnant participants. There was no significant relationship between iodine status and FT 4 or TSH levels in either the pregnant group or non-pregnant group. Separate analysis of patients with diabetes and postpartum women did not significantly alter these results. However, there was a weak correlation between FT 4 and urinary iodine levels when examined as a continuous variable (Pearson correlation coefficient, 0.26; P = 0.016).

The median iodine concentration in the 26 postpartum women (79 µg/L) who provided repeat urine samples for iodine measurement three months after delivery was considerably lower than that in the 81 pregnant women (104 µg/L). However, this difference was not statistically significant ( P = 0.249).