Congenital hypothyroidism (CH) is among the most common metabolic conditions presenting among live births in the United States (US). Multiple subtypes of CH have been identified. Primary CH is the most common form and has been increasing in the US since 1990. If untreated or if treatment is delayed, CH can produce several complications, including disrupting normal neurodevelopment. Little is known about the possible causes of CH.  Previous studies suggest pesticides may influence thyroid function in adults; however, few studies have explored  the role of parental pesticide exposures on thyroid function in offspring. As such, we propose to explore relations between parental pesticide exposures reported by Iowa Agricultural Health Study (AHS) participants on newborn thyroid function and confirmed diagnoses of primary CH in their offspring. The researchers hypothesize that parental exposure to pesticides during pregnancy is positively associated with impaired thyroid function and diagnosis of primary CH in offspring. To test these hypotheses, they aim to: 1) link Iowa AHS participant offspring data with the multi-decade repository of Iowa Newborn Screening (NBS) data and 2) examine associations between parental pesticide exposure during pregnancy reported by AHS participants and thyroid function and confirmed primary CH among their offspring. The proposed design will allow the researchers to link Iowa AHS participant offspring with results of Iowa NBS to describe their newborn metabolic profiles and determine thyroid status. It also will allow the researchers to examine the effects of parental occupational pesticide exposures on newborn thyroid function by comparing thyroid function in offspring of AHS participants to offspring of non-AHS participants whose parents were less likely to have occupational pesticide exposure. Due to the increasing incidence of CH, elucidating the etiologies of primary CH is of great importance. The study will provide further insights into the role of environmental factors on newborn thyroid development and function.