In utero and fetal thyroid health

Having a healthy and functioning thyroid as a pregnant person is necessary for prenatal development — from the moment of conception. This is because the thyroid only starts developing and functioning around the beginning of the second trimester, when the embryo becomes a fetus (1).

Even in the second and third trimester the fetus is dependent on the mother as a source of iodide (1). A pregnant mother is the source of about one third of T4 hormone for the fetus, and the placenta is responsible for converting T4 to T3 hormone (1).

A pregnant mother’s thyroid hormone contribution towards prenatal development is crucial throughout an entire pregnancy. From germinal to embryonic development, to the development of all the fetus’ organs — especially for the formative development of the brain (1).

How are fetal thyroid disorders diagnosed?

  • Ultrasound
    A safe, non-invasive procedure that captures images of the inside of your body. It gives a good estimation of fetal thyroid size, but it’s not great for detecting subtle thyroid changes (2, 3)
  • Cordocentesis
    Measuring hormones through a prenatal umbilical blood sampling. This procedure brings risk of fetal development complications (4–8)

What’s important to know when diagnosing a fetal thyroid disorder?

  • Maternal iodine status (9, 10)
  • Environmental and nutritional conditions
    Especially being aware of any exposure to endocrine-disrupting chemicals or environmental pollutants (2)
  • Maternal thyroid antibody levels
    There is a higher risk thyroid function complications in fetuses of mothers with high TPO and Tg antibodies (2)

How we write: our information is based on the results of peer reviewed studies using the National Library of Medicine platform. It is written by scientists and reviewed by external experts. If you believe we might have overseen crucial scientific information, please contact us at

Disclaimer: This information is not intended to mitigate, prevent, treat, cure or diagnose any disease or condition. If you want to change your treatment, lifestyle, your diet, include supplements in your diet or have concerns about your health, please consult your doctor before trying new approaches.


  1. Fisher DA. Fetal thyroid function: diagnosis and management of fetal thyroid disorders, 1997
  2. Barbosa RM, et al. Ultrasound Measurements of Fetal Thyroid: Reference Ranges from a Cohort of Low-Risk Pregnant Women, 1998
  3. Achiron R, et al. The development of the foetal thyroid: in utero ultrasonographic measurements, 1998
  4. Thorpe-Beeston JG, et al. Fetal thyroid function, 1993
  5. Agrawal P, et al. Intrauterine diagnosis and management of congenital goitrous hypothyroidism, 2002
  6. Morine M, et al. Antenatal diagnosis and treatment of a case of fetal goitrous hypothyroidism associated with high-output cardiac failure, 2002
  7. Tongsong T, et al. Fetal loss rate associated with cordocentesis at midgestation. American Journal of Obstetrics and Gynecology, 2001
  8. Liao C, et al. Efficacy and safety of cordocentesis for prenatal diagnosis, 2006
  9. Zimmermann MB. Iodine deficiency in pregnancy and the effects of maternal iodine supplementation on the offspring: a review, 2009
  10. Pearce EN, et al. Consequences of iodine deficiency and excess in pregnant women: an overview of current knowns and unknowns, 2016

Photo: Unsplash; Design: VLM Health



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