It is suggested that organic fed cows consume much higher levels of goitrogens in their diets. That ends up with reduced iodine uptake, hence less iodine to go into the milk.
I am not entirely convinced of the mechanism but the idea of it being generally lower iodine content appears well founded. Here is one significant study:
Br J Nutr. 2012 Apr;107(7):935-40. doi: 10.1017/S0007114511003059. Epub 2011 Jul 5.
Iodine concentration of organic and conventional milk: implications for iodine intake.
Bath SC1, Button S, Rayman MP.
1Nutritional Sciences Division, Faculty of Health and Medical Sciences, University of Surrey, Guildford GU2 7XH, UK.
Iodine is required for adequate thyroid hormone production, which is essential for brain development, particularly in the first trimester of pregnancy. Milk is the principal source of iodine in UK diets, and while small studies in Europe have shown organic milk to have a lower iodine concentration than conventional milk, no such study has been conducted in Britain. In view of the increasing popularity of organic milk in the UK, we aimed to compare the iodine concentration of retail organic and conventional milk and to evaluate regional influences in iodine levels. Samples of organic milk (n 92) and conventional milk (n 80), purchased from retail outlets in sixteen areas of the UK (southern England, Wales and Northern Ireland), were analysed for iodine using inductively coupled plasma MS. The region of origin of the milk was determined from information on the label. Organic milk was 42·1 % lower in iodine content than conventional milk (median iodine concentration 144·5 v. 249·5 ng/g; P < 0·001). There was no difference in the iodine concentration of either conventional or organic milk by area of purchase. However, a difference was seen in iodine concentration of organic milk by region of origin (P < 0·001). The lower iodine concentration of organic milk has public-health implications, particularly in view of emerging evidence of iodine deficiency in UK population sub-groups, including pregnant women. Individuals who choose organic milk should be aware that their iodine intake may be compromised and should ensure adequate iodine intake from alternative sources.