Mature Human Milk Macronutrient Composition and Apparent Milk TSH Concentration in Women with Treated Hypothyroidism Attending Lactation Counseling: A Cross-Sectional Study

Background/Objectives: Hypothyroidism may influence lactation physiology, but evidence regarding its association with mature human milk composition remains limited. This cross-sectional study evaluated macronutrient composition and apparent thyroid-stimulating hormone (TSH) concentration in 24 h composite mature milk samples from women with a documented history/diagnosis of treated or clinically managed hypothyroidism attending lactation counseling and human milk-bank consultation, compared with controls without known 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 during routine preconception or early-pregnancy/perinatal care; the study did not include contemporaneous maternal serum TSH/FT4 testing at milk collection in either group. Composite 24 h milk samples were analyzed using the MIRIS Human Milk Analyzer for fat, crude protein, true protein, carbohydrates, total solids, and energy. Milk TSH was measured using a third-generation ELFA assay. Because this serum/plasma assay was not formally validated for the human milk matrix, values are reported as exploratory apparent milk TSH concentrations. Results: No statistically significant between-group differences were observed in macronutrient composition in the analyses performed. For example, the HG-CG mean difference for energy content was −3.47 kcal/100 mL (95% CI: −8.45 to 1.52; p = 0.169; q = 0.814), and all confidence intervals for milk-composition outcomes included zero. Median apparent milk TSH concentration was 0.0180 [IQR 0.0130–0.0208] µIU/mL in controls and 0.0155 [IQR 0.0093–0.0210] µIU/mL in the hypothyroidism group (Mann–Whitney U = 459.0, p = 0.286). Hyperlactation was frequent in this selected cohort and was more common 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, a documented diagnosis/history of treated or clinically managed hypothyroidism was not associated with statistically significant differences in mature milk macronutrient composition or apparent milk TSH concentration. These findings are exploratory and should not be interpreted as evidence of equivalence, confirmed euthyroidism at sampling, or exclusion of smaller effects.