Study Finds No Colostrum Nutrient Changes in Treated Hypothyroidism

A prospective cohort study published in Frontiers in Nutrition reports that well-controlled maternal hypothyroidism treated with levothyroxine during pregnancy does not alter the macronutrient composition or energy content of colostrum in the first five days postpartum. The research, led by Joanna Maria Jassem-Bobowicz from the Medical University of Gdańsk in Poland, involved 120 postpartum women, comprising 45 with well-controlled hypothyroidism and 75 healthy controls. The study aimed to address a gap in scientific literature regarding the specific effects of adequately managed thyroid dysfunction on early human milk composition, a topic that has previously yielded variable and limited findings.

Maternal hypothyroidism is a common endocrine disorder during pregnancy, affecting an estimated 3 to 5.5% of pregnant women, and is often associated with adverse maternal and neonatal outcomes if left untreated. While untreated or poorly controlled hypothyroidism may influence lactation and milk composition, the impact of well-controlled disease on human milk remained insufficiently characterized prior to this investigation. The study specifically focused on colostrum, the first form of milk produced in the days after birth, which differs substantially in composition from mature milk. By isolating women with clinically euthyroid status, the researchers sought to determine if adequate treatment neutralizes potential metabolic effects on early milk nutrients.

The study design included the collection of colostrum samples within the first five days after delivery from all participants. Researchers utilized mid-infrared spectroscopy, specifically the MIRIS® system, to analyze the samples. This technology allows for the quantification of macronutrients, including lipids, crude and true protein, carbohydrates, total solids, and energy content, measured in grams per 100 milliliters and kilocalories per 100 milliliters, respectively. The MIRIS® system was calibrated daily and checked regularly to ensure accuracy, and samples were homogenized using an ultrasonic processor immediately before analysis to ensure consistent measurement of fat and other components.

Statistical analysis was performed using the Mann–Whitney test for continuous variables and Fisher’s exact test for binary variables, given the non-normal distribution of the data. Effect sizes were assessed using Cliff’s delta, with 95% confidence intervals estimated through bootstrap resampling. The study did not conduct an a priori power analysis, but a post hoc sensitivity analysis indicated that the sample size provided approximately 80% power to detect a moderate standardized between-group difference of about 0.53. This statistical context is important for interpreting the null findings, as the study was adequately powered to detect moderate or larger differences but could not reliably exclude smaller biological variations.

The results showed no statistically significant differences between the hypothyroid and control groups in any measured component of milk composition. For lipids, the median concentration was 2.5 g/100 mL in the hypothyroid group and 2.6 g/100 mL in the control group, with a p-value of 0.32. Crude protein levels were 2.0 g/100 mL in the hypothyroid group and 2.1 g/100 mL in controls, while true protein was 1.6 g/100 mL in both groups. Carbohydrate concentrations were 7.9 g/100 mL in the hypothyroid group and 7.8 g/100 mL in controls. Total solids and energy content also showed no significant differences, with energy levels at 64 kcal/100 mL in the hypothyroid group and 63 kcal/100 mL in controls. All p-values were greater than 0.05, and effect sizes were negligible, ranging from −0.11 to −0.07.

In addition to macronutrient analysis, the study examined neonatal outcomes and maternal characteristics to ensure appropriate clinical management of the participants. There were no significant differences in maternal age, pre-pregnancy weight, body mass index, or parity between the two groups. Neonatal characteristics, including gestational age, birth weight, head circumference, body length, and Apgar scores, were also similar between the groups. The similarity in these clinical parameters supports the conclusion that the hypothyroid group was well-managed during pregnancy, with thyroid-stimulating hormone and free thyroxine levels maintained within pregnancy-specific reference ranges according to Polish Endocrine Society guidelines.

An exploratory analysis investigated the association between levothyroxine dosage and macronutrient content within the hypothyroid group. The average daily dose of levothyroxine before delivery was 89 micrograms, with a range of 25 to 235 micrograms. Kendall’s tau correlation coefficients indicated weak positive associations between dosage and macronutrient levels, ranging from 0.01 to 0.10, but none were statistically significant. The researchers cautioned that levothyroxine dose reflects individual replacement needs and clinical management rather than direct maternal thyroid hormone exposure, so these associations should be interpreted with caution and do not imply a causal relationship between dosage and milk composition.

The study also explored how macronutrient composition changed with gestational age. While protein, lipids, and energy content did not differ significantly across gestational ages in either group, carbohydrate concentration remained constant in the hypothyroid group. In contrast, a weak positive association between carbohydrate concentration and gestational age was observed in the healthy control group. This finding arose from an exploratory analysis without adjustment for multiple comparisons and requires confirmation in larger studies. Carbohydrates are generally considered stable components of human milk, but previous research has reported gestational-age-related differences in other milk components, such as oligosaccharides.

Several limitations of the study must be considered when interpreting the results. First, the study only analyzed colostrum from the first five postpartum days and did not assess mature milk composition, which changes significantly over time. Second, the MIRIS® technology used in this study cannot determine fatty acid profiles, micronutrient content, oligosaccharides, hormones, immune factors, or bioactive proteins. Therefore, the absence of differences in total lipid concentration does not exclude alterations in the quality or specific types of fatty acids, which have been reported in other studies of hypothyroid women. Third, the study may have experienced selection bias related to milk availability, as samples were only collected from mothers with sufficient colostrum to meet their infant’s nutritional needs. This approach may have excluded women with delayed lactogenesis or insufficient milk production, potentially limiting the generalizability of the findings to all women with hypothyroidism.

Furthermore, the study did not stratify participants by the underlying etiology of hypothyroidism, such as autoimmune versus non-autoimmune causes, due to limited sample sizes. The researchers also did not systematically assess maternal factors that could influence milk composition, including diet, iodine and selenium status, DHA supplementation, or postpartum hormonal status. These factors were not available in the routine clinical records and could not be accounted for in the statistical analyses. The collection window of days 1 to 5 postpartum may have contributed to variability in the data, as colostrum composition changes rapidly during this period, and most samples were collected on days 4 or 5 when milk availability was greater.

The findings of this study are reassuring for women with well-controlled hypothyroidism, suggesting that their early colostrum provides nutritionally adequate macronutrients comparable to those of healthy mothers. However, the results should not be extrapolated to breastfeeding performance, lactational volume, or long-term lactational outcomes. The study was not designed to assess the safety of breastfeeding or the effects of hypothyroidism on pregnancy outcomes, although the similar neonatal characteristics observed suggest adequate maternal disease control. The negligible effect sizes observed indicate that any undetected differences are unlikely to be large, but larger longitudinal studies with broader compositional analyses are warranted to confirm these findings and investigate the impact of hypothyroidism on bioactive components and mature milk.

This research contributes to the growing body of evidence on the intersection of maternal endocrine health and infant nutrition. By focusing on a well-defined population with clinically managed hypothyroidism, the study provides specific insights that differ from previous reports involving uncontrolled or poorly managed thyroid dysfunction. The study underscores the importance of adequate thyroid hormone replacement in maintaining normal early milk composition, while highlighting the need for further research into the complex bioactive aspects of human milk that are not captured by macronutrient analysis alone.

Subject of Research: Well-controlled hypothyroidism does not affect the macronutrient composition of colostrum

Article Title: Well-controlled hypothyroidism does not affect the macronutrient composition of colostrum

Article References: Jassem-Bobowicz, J. M., Pastuszak, B., Sokołowska, E. M., Drążkowska, I., Narkowicz, S., & Domżalska-Popadiuk, I. (2026). Well-controlled hypothyroidism does not affect the macronutrient composition of colostrum. Frontiers in Nutrition, 13, Article 1921550. https://doi.org/10.3389/fnut.2026.1921550

Image Credits: AI Generated

DOI: 10.3389/fnut.2026.1921550

Keywords: Well-controlled, hypothyroidism, does, affect, macronutrient, composition, colostrum, scientific research