The purpose of this dissertation was to investigate the efficacy of iodine supplementation vs. placebo, in reproductive-age women, 18-45 years, in improving iodine status, thyroid function, resting metabolic rate, and body composition in a six-month, randomized-double-blinded-placebo-controlled trial. Non-pregnant (euthyroid, normal thyroid stimulating hormone (TSH), mean=1.57mIU/mL) women were randomized into two groups: 12.5mg Iodoral® (IG, n=65) or placebo (PG, n=38). Assessments included iodine status determination (24-hr urine iodine (UI), %-iodine saturation (% IS), sodium-iodide-symporter-ratio (NIS), saliva and serum iodide concentrations), thyroid function (serum TSH, free-thyroxine (T4), and free-tri-iodothyronine (T3) concentrations), body composition analysis using Dual Energy X-ray Absorptiometry (DXA), resting metabolic rate (RMR) testing, and analysis of 3-day dietary records, health, demographic, and physical activity questionnaires. Analysis of the data revealed dietary iodine intake to be significantly below standard recommended dietary allowance (RDA) of 150 µg iodine/d for IG and PG at baseline and six months. For the first time, associations were observed between dietary iodine intake and body composition, with decreased dietary iodine intake being associated with higher body fat content (p<0.01). Iodine status indicators, 24-hr UI and % IS were also significantly below normal, indicating iodine deficiency in the study population. Although 24-hr UI and % IS for IG and PG showed an increased trend from baseline to six months, statistical significance was not observed for between and within group effects, indicating that a longer duration of supplementation may be needed to improve iodine status in deficient populations. Saliva iodide increased significantly in IG (p=0.041), and PG (p=0.013) at the end of six months; however, NIS ratio remained unchanged, indicating normal functioning of the NIS. Free-T4 increased significantly at six months in IG and PG (p<0.001), however other thyroid function parameters remained unchanged, indicating that the high dose iodine supplement may be better tolerated than expected. RMR significantly increased in IG and PG (p<0.001) at six months, and was positively correlated (p<0.01) with all body composition variables. Overall, participants demonstrated a generalized lack of awareness of iodine nutrition and the implications of iodine deficiency in reproductive-age women, indicating a significant public health concern that needs to be addressed. Advisors/Committee Members: DiMarco, Nancy M (advisor), Petterborg, Larry (committee member), Warren, Cynthia (committee member), Maziarz, Mindy (committee member), Basiliadis, Margaret (committee member).