Iron is essential for the normal functioning of thyroid peroxidase (TPO-abs) - a protein essential for the correct functioning of the thyroid. Pregnant women need to make enough thyroid hormone for the full development of their babies' brains, which is especially critical during the first semester when the foetus has not developed a thyroid gland of its own, said the paper.
Iron deficiency also causes thyroid autoimmunity - a disease where the immune system mistakenly destroys healthy thyroid cells; causing thyroid hormone levels to fall. It can be particularly dangerous for pregnant women.
For the study, the team followed 1900 pregnant women who were in their first trimester.
They measured the women's blood ferritin - an indicator of iron deficiency - antibodies against the thyroid peroxidase - indicating thyroid autoimmunity -, the thyroid hormone free thyroxine (FT4) and thyroid stimulating hormone (TSH).
A third or 35 per cent of the study participants showed iron deficiency.
"Considering that our study took place in a relatively wealthy country, our results show that even in 2016, iron deficiency remains an important problem," said lead author Kris Poppe from Saint-Pierre University Hospital in Brussels.
The results were published in European Journal of Endocrinology.