Postpartum women experience abnormalities in thyroid function at twice the prevalence of the general population [1]. Thyroid hormone is essential in maintaining the human body’s metabolism, temperature (thermoregulation), and psychological mood [2]. In a developing brain, thyroid hormone is responsible for adequate myelination, neuron cell maturation, and central nervous system development [3]. Optimal thyroid function relies on adequate biosynthesis of thyroid hormones, which depends on three dietary minerals: iodine, selenium, and iron [4,5]. Pregnancy increases thyroid hormone turnover; thus, women with limited thyroidal reserve or marginal iodine deficiency are at increased risk to develop thyroid dysfunction after birth [6]. This is one of the most common endocrine disorders that postpartum women experience [7].

Iodine is the major component of thyroid hormones, but it is also a regulator for the synthesis and secretion of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). Selenium, as a component of the selenocysteine-containing proteins glutathione peroxidase, protects the thyroid gland from oxidative damage [5]. Selenoproteins are required to convert T4 to T3, the active form of thyroid hormone. Iron is required for heme-dependent thyroperoxidase activity, which is required for the synthesis of adequate thyroid hormone. Selenium deficiency and iron deficiency anemia may negatively affect thyroid hormone synthesis by impairing selenium- and iron-dependent enzyme activities, even if iodine status is adequate [5]. Most previous research has investigated iodine, selenium, and iron intake/status separately or a combination of any two of them among women of childbearing age [8] and postmenopausal women [9]. However, further research is needed to explore all three micronutrients together, acknowledging their close relationship in thyroid hormone synthesis.

Thyroid dysfunction is a significant health issue in New Zealand, with women diagnosed at 5 times the prevalence in men [10,11]. Concerning adequate thyroid function, iodine, selenium, and iron play important roles. In New Zealand, soils provide low levels of available iodine and selenium, resulting in low concentrations in the food supply [12], hence in the diet [13]. Iodine deficiency in early life is associated with impaired neurodevelopment [14]. Iodine deficiency was a concern in New Zealand in the early years of the 20th century, but its prevalence was mostly reduced through the introduction of iodized salt in the 1930s. However, since the 1990s, a number of studies in New Zealand have shown iodine deficiency has reemerged in adults [15], pregnant and breastfeeding women [16,17], school children [18] and breastfed infants and toddlers [19]. To improve iodine status in New Zealand, two government initiatives were introduced: mandatory iodized salt in commercially made bread products from September 2009 and the provision of iodine supplementation for all pregnant and lactating women in 2010 [20]. Although recent studies suggested that adults [21,22] and children [23] in New Zealand may now have adequate iodine intake/status, both pregnant and breastfeeding women remain deficient. A pilot study of a small sample of self-selected highly educated pregnant and breastfeeding women assessed urinary iodine excretion, breast milk iodine concentration, and blood thyroglobulin and suggested iodine deficiency [24]. There is a need for a more robust investigation into the iodine status of postpartum women and their infants from a wide range of socioeconomic backgrounds.

Low selenium status in New Zealand has been partially reversed by increased consumption of imported flour from Australia (which generally has higher selenium concentrations than flour produced in New Zealand) [25,26]. In addition, both pregnant and breastfeeding women have an increased requirement for dietary selenium due to the demands from the fetus and breastfed infants. Previous research, which investigated selenium status among postpartum women and their infants in New Zealand 20 years ago [27], measured urinary selenium excretion and plasma selenium and indicated that such women were at risk of selenium deficiency. To our knowledge only one small study of breastfeeding women, by our research group, assessed dietary selenium, urinary selenium excretion, and breast milk selenium concentration and suggested selenium inadequacy was still a concern [28]. There remains a lack of research investigating selenium status among postpartum women and their infants.

Health professionals closely monitor the iron status of women during pregnancy. However, after birth, management of iron status can be inconsistent. Results from a UK multicenter study reported only 50% of postpartum women had hemoglobin levels checked after delivery (with 30% of those women confirmed as anemic), while the overall iron stores of participating women remained unexamined [29]. Generally, postpartum women’s iron status recovers as a consequence of cessation of menstrual bleeding since conception or a minimal secretion of iron via breast milk if breastfeeding [30]. However, if women have suffered iron deficiency before and/or during pregnancy and/or have experienced significant blood loss during the birth, their iron status may not reach optimal levels even if an intervention subsequently occurs. A New Zealand study of 186 women found 77% of women were not tested for hemoglobin levels after birth. Further, out of those most at risk (with low iron status during late pregnancy and high blood loss, exceeding 500 mL, during birth), few women were then retested for their iron status after 10 days postpartum [31]. Iron status of postpartum women remains largely underreported.

Moreover, low serum selenium has been identified as an independent risk factor for depression, [32] and selenium supplementation has been observed to reduce postnatal depression [33]. Postnatal depression is one of the main disorders women experience postnatally, its onset being timed at 6 weeks to 6 months after birth. Most women will recover from postpartum depression, though approximately one-quarter of affected women report being depressed when their infant reaches their first birthday [34]. Using the measured criteria of postnatal depression on the Edinburgh Postnatal Depression Scale (EPDS), the prevalence of postnatal depression in New Zealand was about 8% in 1994 and 16% in 2006. In the 2015 New Mothers’ Mental Health Survey [35], the prevalence was 14% and is now recorded as the most common disorder for mothers in their first year after childbirth [36].

Of additional concern, mothers are often reluctant or unable to seek help when they experience symptoms of postnatal depression [35]. Such underdiagnosed and untreated mental health conditions affect both the mother and their children’s ongoing cognitive, emotional, and behavioral development [37]. Despite other social and psychological etiology of depression, potential links between micronutrient status, thyroid hormone, and the risk of postpartum depression need to be further explored. This may help develop new preventive approaches to lowering the risks of postpartum depression.

The study’s primary outcomes include (1) investigating breastfeeding women’s iodine intake and status among supplement users and nonusers following the implementation of two government initiatives to improve iodine status; (2) examining maternal iodine, selenium, and iron intake status; and (3) exploring iodine, selenium, and iron status in maternal thyroid function.

In addition, the study provides preliminary data on possible associations between women’s selenium status, thyroid function, and postnatal depression over a 1-year period and infants’ iodine and selenium status in relation to neurodevelopment during their first year of life. Ultimately, this research will inform a future larger study of potential variables impacting maternal thyroid function and the risk of postnatal depression, together with early infant neurodevelopment.

The Mother and Infant Nutrition Investigation (MINI) study is an observational longitudinal cohort study spanning the first year postpartum. It was approved by the Health and Disability Ethics Committee (15/NTA/172) in December 2015. The study’s ethics approval was registered with the Royal New Zealand Plunket Ethics Committee in June 2016. The MidCentral District Health Board in New Zealand also approved the study. The study was registered with the Australian New Zealand Clinical Trials Registry [ACTRN12615001028594].

The study is being conducted in the Human Nutrition Research Unit at Massey University, Palmerston North, New Zealand. The first study visit for participants is at approximately 3 months postpartum, and follow-up assessments take place at 6 months and 12 months postpartum (Multimedia Appendix 1).

The target population for the study was healthy breastfeeding women aged over 16 years who had birthed a healthy term singleton infant 3 months prior. Women were excluded if they developed significant health problems, such as metabolic disease or cancer. Women were excluded if they had been diagnosed or treated at any time for hyperthyroidism or hypothyroidism. Participants were required to live within or near the local Palmerston North area and be able to attend Massey University for scheduled study visits. Women of any ethnic and socioeconomic status were eligible.

Posters to promote the study were placed at selected sites (general practitioner surgeries, midwifery clinics, pharmacies, antenatal classes, ultrasound clinics, maternal wards in hospitals, local community playgroups, and early childhood centers, etc). Local newspapers and social media sites were used to publicize the study. Local midwives, childbirth educators, and lactation consultants were asked to raise awareness of the MINI study to their clients. An effort was made to recruit women from a wide range of socioeconomic backgrounds and ethnic groups, including Maori, Pacific Islanders, and Asian women. Potential participants responded by recording an expression of interest online or via telephone or email. Prospective participants were provided with a study information sheet. Interested participants then completed a screening questionnaire to ensure eligibility. Written informed consent was obtained from all participants before their enrollment in the study. Mothers also gave written consent to their infants’ participation in the study. After providing informed consent, participants were assigned a unique identifier code and scheduled for their first study visit.

The main outcome measure was iodine excreted per day, and the sample size was calculated using G*Power 3.1 (Heinrich Heine University) based on data (mean and standard deviation) from a preliminary study of breastfeeding women [24]. Calculation used 1-way analysis of variance with two groups (95% power, 𝛼=.05, 2-tailed) and three repeat measures; 80 participants were needed, using expected mean daily urine iodine concentrations of 140 and 100 µg/d for iodine supplement users and nonusers, respectively, and a standard deviation of 60.

Statistical analysis will be performed using SPSS Statistics version 23. The Shapiro-Wilk test will be used to test for data normality. Nonparametric data will be expressed as median (25th, 75th percentile), and parametric data will be expressed as mean and standard deviation. Bivariate correlations will be tested using the nonparametric Spearman 𝜌 correlation coefficient. Repeated-measures analysis of variance will be used to calculate continuous variables between groups. Nonparametric Mann-Whitney U test (2-tailed) will be used to examine iodine intake and status between supplement users and nonusers. Multiple regression models analysis will be used to determine the associations between iodine, selenium, iron status, and thyroid function, as well as considering confounding factors. Multivariate analysis will be used to examine possible associations between women’s selenium status, thyroid function, and postnatal depression and infant first year neurodevelopment.

A unique aspect of this study is that it will investigate all three micronutrients responsible for adequate thyroid hormone synthesis concurrently, rather than each separately in isolation. This observational longitudinal cohort study will measure the iodine and selenium status of women repeatedly in their first year after birth, which provides an evaluation of their nutritional status. Iodine status among supplement users and nonusers will provide up-to-date data on this postpartum group in New Zealand around 8 years after government interventions. Results will explore whether maternal iodine and selenium status could be used as a proxy measure of infant status. It provides an opportunity to examine the association of maternal iodine and selenium with infant neurodevelopment during their first year. This study explores selenium status using both short-term and long-term measures in relation to neurodevelopment at 6 months and 12 months of age, which has not been reported previously. Furthermore, the study results will add preliminary data on iron status of women at 6 months postpartum.

Importantly, the study will investigate overall thyroid function of women at 6 months postpartum with respect to the risk of postnatal depression. Measurement of thyroid hormones, thyroid stimulating hormone, anti-TPO, Tg, and anti-Tg in serum as well as measuring thyroid gland volume via ultrasound will provide an overall picture of maternal thyroid function after giving birth. This is an opportune time to check thyroid status, especially as women with limited thyroidal reserve or iodine deficiency in pregnancy may develop postpartum thyroid dysfunction, one of the most common endocrine disorders women experience [6,58].

Additionally, there will be longitudinal assessment of mothers’ mental health via repeated screening by using the EPDS. The results may add to the literature in postpartum mental health status. Their offsprings’ growth and neurodevelopment will be followed during the first year after birth. The findings from this study have the potential to inform future public health policy and practice regarding postpartum women’s nutritional status and mental health together with infant health outcomes.