Background: According to the WHO estimates, there are approximately 2 billion people suffering from iodine deficiency. Inadequate iodine intake may result in varying degrees of adverse health outcomes ranging from mild hypothyroidism, which is associated with adverse pregnancy outcomes, to foetal iodine deficiency that impairs neurocognitive development of the growing foetus. However, the extent of ethnic differences in thyroid diseases, as well as the contributory factors in early pregnancy are still unknown. The overall aim of the present thesis is to evaluate iodine status and its effects on thyroid function during 2nd and 3rd trimesters of pregnancy and 3 months post-partum in a multi-ethnic cohort of pregnant women who participated in the STORK Groruddalen Study. Materials and Methods: This thesis is based on a retrospective cohort study; the STORK Groruddalen study, which is a population-based cohort study in women attending gestational health controls at the Child Health Clinics (CHC) in the areas Bjerke, Stovner and Grorud in Oslo during 2008-2010. Groruddalen has a multi-ethnic population (40-50% with ethnic minority background) with high attendance rate at the CHC (75-85%). High quality data was collected at gestational week (GW) 14, 18 and at 3 months post-partum. Thyroid Stimulating Hormone (TSH) and Free Thyroxine (FT4) were assayed with in-house methods at Akershus University Hospital and the Hormone Laboratory, Oslo University Hospital, respectively. Urinary iodine was assayed in spot urine by a colorimetric method at the Hormone Laboratory, Oslo University Hospital (n = 681). Results: Generally, TSH levels significantly increased between 1st trimester (1.99 ± 1.42 mIU/L) and 2nd trimester (2.42 ± 1.46 mIU/L) and then decreased at 3 months post-partum (2.31 ± 3.03 mIU/L). FT4 levels showed an opposite trend with downward shift from 1st trimester (14.8 ± 2.29 pmol/L) to 2nd trimester (12.9 ± 1.70 pmol/L), and later an upward inclination at 3 months post-partum (14.7 ± 2.62 pmol/L). All ethnic groups showed similar pattern during pregnancy, however, South Asians showed increased TSH at 3 months post-partum (3.41 ± 5.38 mIU/L). The median urinary iodine concentration (UIC) was 88.8 μg/L. 78.8% of the study sample had UIC < 150 μg/L, and more 70% of the population in each ethnic group was iodine deficient. East Asian women had the highest median UIC (108 μg/L; IQR: 60, 152). Pregnant women with South Asian ethnicity had higher and wider TSH range (0.02 ± 8.06 mIU/L) during 1st trimester with significantly increased TPO-Ab positivity. Conclusion: The findings demonstrate a normal physiological pattern of TSH and FT4 levels during pregnancy and post-partum in the study population. South Asian pregnant women showed higher TSH levels which may increase the risk of developing thyroid dysfunction. Moreover, other ethnic groups also had suboptimal TSH levels with regard to the recommended TSH ranges during pregnancy. The results of median UIC also indicates that the pregnant women in the study population are iodine deficient which can incur additional risk for thyroid dysfunction.