Abstract
<jats:p>Background/Objectives: Thyroid disorders, particularly hypothyroidism and Hashimoto’s disease, are common among women of reproductive age and may influence lactation physiology. Evidence regarding their association with mature human milk composition remains limited. This cross-sectional study eval-uated macronutrient composition and thyroid-stimulating hormone (TSH) con-centration in 24-hour composite mature milk samples from women with diag-nosed and clinically managed hypothyroidism attending lactation counseling, compared with controls without thyroid disorders. Methods: Sixty-six lactating women attending the Human Milk Bank in Toruń, Poland, were enrolled (31 with hypothyroidism, including six with Hashimoto’s disease; 35 controls). Thyroid characterization was based on diagnoses and laboratory results documented dur-ing routine preconception or early-pregnancy/perinatal care; the study did not in-clude contemporaneous maternal serum TSH/FT4 testing at milk collection. Com-posite 24-hour milk samples were analyzed using the MIRIS Human Milk Analyz-er for fat, crude protein, true protein, carbohydrates, total solids, and energy. Milk TSH concentrations were measured using a third-generation ELFA assay; because the assay was not formally validated for the human milk matrix, these values should be interpreted as exploratory apparent milk TSH concentrations. Results: No statistically significant differences were observed in macronutrient composi-tion or milk TSH concentrations between groups in the analyses performed. Hy-perlactation was frequent in this selected counseling cohort and was more com-mon in controls (27/35; 77.1%) than in the hypothyroidism group (16/31; 51.6%). Exploratory analyses of pre-pregnancy BMI and Hashimoto’s disease were limited by small subgroup sizes. Conclusions: In this small selected cohort, diagnosed and clinically managed hypothyroidism was not associated with statistically signifi-cant differences in mature milk macronutrient composition or milk TSH concen-tration. These findings are exploratory and should not be interpreted as evidence of equivalence, confirmed euthyroidism at sampling, or absence of any possible biological effect.</jats:p>