New mothers on thyroid pills often wonder if their first milk is 'weaker'. In 120 Polish mothers, it was not

An underactive thyroid is one of the most common hormone problems in pregnancy. The authors of a new study put it at 3 to 5.5% of pregnant women, and most are treated with levothyroxine, a daily tablet that replaces the missing hormone. Thyroid hormones help the breast prepare for milk, and earlier studies have hinted that untreated or poorly controlled hypothyroidism can change what is in the milk. That leaves treated mothers with an obvious question: is my first milk as good as everyone else's? A study published on 29 October in Frontiers in Nutrition looked at exactly that early milk, colostrum.

The team works at the neonatology department and human milk bank of the University Clinical Center in Gdańsk, Poland. Between March 2024 and April 2026 they recruited 120 women who had just given birth there: 45 whose hypothyroidism had been treated and kept within pregnancy target ranges, and 75 with normal thyroid function. Smokers, women with diabetes and women whose babies had recently had formula were left out. Each mother gave up to 15 mL of colostrum in the first five days after birth, mostly on day four or five, collected between 8 a.m. and 2 p.m. The samples were coded so the people analysing them did not know which group they came from, then run through a mid-infrared milk analyser two or three times each.

The results were strikingly similar. Median fat was 2.5 g per 100 mL in the thyroid group and 2.6 g in the comparison group; crude protein was 2.0 and 2.1 g; carbohydrate 7.9 and 7.8 g; energy 64 and 63 kcal per 100 mL. None of the differences came close to statistical significance, and the effect sizes were described as negligible. Within the thyroid group, the levothyroxine dose before delivery averaged 89 micrograms a day (range 25 to 235), and a bigger or smaller dose was not linked to any of the milk measures. The babies were also similar in birth weight, length, head size and Apgar scores.

The authors are careful about what this does not show. To protect feeding, they only took samples from mothers who had milk to spare, so women whose milk came in late or in small amounts, a problem that has been linked to thyroid trouble, were not in the study. The analyser measures only fat, protein, sugar and calories, not fatty acid types, vitamins, hormones or immune factors, and only colostrum was tested, not mature milk. The study was large enough to spot a moderate difference between groups but not a small one. One exploratory finding, that milk sugar rose slightly with pregnancy length in the comparison group but not in the thyroid group, needs confirming. Within those limits, it is a reassuring result for mothers whose thyroid is well managed.