Second and third trimester of pregnancy with an underactive thyroid

During second and third trimester of pregnancy all of the fetus’s organs will rapidly develop in preparation for birth.

Thyroid glands of both the mother and fetus play a major role in all the developmental stages, and it’s necessary to make sure the thyroid and immune system are properly functioning during these stages.

Thyroid and fetus development

Your fetus is dependent on your thyroid hormones during the first trimester of pregnancy. The fetus thyroid starts producing hormones during the second trimester of pregnancy, and it becomes fully functional during the third trimester. Until the fetus’s thyroid gland can produce all of its own T4, it needs the help of the mother’s thyroid hormones to properly develop (1).

Thyroid function testing

There is no universally accepted thyroid function screening for pregnant people, but there is a recommendation for thyroid function testing if you (2,3):

  • Are living in an area with iodine insufficiency
  • Are experiencing symptoms of hypothyroidism
  • Have a family history of thyroid disease (including Hashimoto’s)
  • Are diagnosed with type 1 diabetes
  • Have a history of head and neck radiation
  • Experiences recurrent miscarriages
  • Are obese

Thyroid hormone ranges

Generally accepted ranges* for TSH are (3):

  • Second trimester: 0.2–3.0 mIU/L
  • Third trimester: 0.3–3.0 mIU/L

*Keep in mind there are individual lab differences.

Free T4 (fT4) and fT3 rise in the first trimester and decrease during the second trimester, but they should stay within the reference range (4). Total T4 (tT4 or T4) will increase throughout the pregnancy, this increase is caused by the hormone estrogen. tT4 can increase 1.5x during the second trimester (2).

Second and third trimester complications

Even when the fetus’s thyroid starts making hormones, the mother’s thyroid hormone levels are still closely correlated. Not having enough thyroid hormones is connected with increased risks of (5–9):

  • Preterm birth — birth given before 37th week of pregnancy
  • Preeclampsia — high blood pressure and signs of damage to an organ system (most often the liver and kidneys), usually starting after 20 weeks of pregnancy in people whose blood pressure had been normal
  • Poorer neurodevelopment in children
  • Second trimester miscarriage
  • Stillbirth

Preterm birth

Preterm birth rates have risen in recent decades. Having an underactive thyroid or Hashimoto’s increases the risk of preterm delivery (10,11), and the risk increases with rising amounts of thyroid antibodies (12,13).

One possible reason for preterm delivery is the high demand for both T3 and T4 in the uterus. The uterus is a muscle which needs a constant and high level of thyroid hormones to prevent it from having unnecessary spasms, and low T4 and T3 causes muscle weakness, which can disrupt the stability of the uterus (14, 15).

A reason is that having an underactive thyroid with high thyroid antibodies is connected to the risk of water breaking too early (16).


Research thus far has mostly focused on levothyroxine therapy. Levothyroxine was shown to reduce the number of miscarriages and stillbirths (17).

Keeping T4 levels in the upper normal ranges — or adding progesterone supplementation — might help delay preterm birth, even in the case of high TPO and TG antibodies, and a seemingly normally functioning thyroid gland (11, 12, 18–20).

There is not yet research on the natural thyroid preparations and advanced stages of pregnancy.

Iodine intake remains especially important during pregnancy, since iodine is the building part of thyroid hormones.

Make sure to talk to your doctor. Know which foods and medications to take, and which to avoid. Some known foods that might impair T4 function are: iron supplements, calcium, soy milk, and cholesterol lowering medication cholestyramine (2).

If your baby is born with an underactive thyroid or Hashimoto’s

If you have hypothyroidism — or have a family history thyroid disorders — your newborn will likely be tested. If needed, your newborn will put on thyroid hormone replacement therapy (21).

A paediatrician will do regular follow-ups — to do test, to ensure thyroid function has stabilized through therapy, and to check that the child is developing normally.

TSH reference values for newborn babies *(21):

  • Up to 1 month of age: 0.5–8.7 mU/mL
  • 1 month–1 year: 0.4–6.3 mU/mL
  • Keep in mind there are individual lab differences.

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. Smith A, et al. Thyroid disorders in pregnancy and postpartum, 2017
  2. Lazarus JH. Thyroid function in pregnancy, 2011
  3. Women’s Health Committee; The Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Testing for hypothyroidism during pregnancy with serum TSH, 2015
  4. Brent GA. Maternal thyroid function: interpretation of thyroid function tests in pregnancy, 1997
  5. Medici M, et al. Maternal early pregnancy and newborn thyroid hormone parameters: The generation R study, 2012
  6. Wilson KL, et al. Subclinical thyroid disease and the incidence of hypertension in pregnancy, 2012
  7. Schneuer FJ, et al. Association and predictive accuracy of high TSH serum levels in first trimester and adverse pregnancy outcomes, 2012
  8. Abalovich M, et al. Overt and subclinical hypothyroidism complicating pregnancy, 2002
  9. Allan WC, et al. Maternal thyroid deficiency and pregnancy complications: implications for population screening, 2000
  10. Chang HH, et al. Born Too Soon preterm prevention analysis group. Preventing preterm births: analysis of trends and potential reductions with interventions in 39 countries with very high human development index, 2013
  11. Sheehan PM, et al. Maternal Thyroid Disease and Preterm Birth: Systematic Review and Meta-Analysis, 2015
  12. He X, et al. Thyroid antibodies and risk of preterm delivery: a meta-analysis of prospective cohort studies, 2012
  13. Ghafoor F, et al. Role of thyroid peroxidase antibodies in the outcome of pregnancy, 2006
  14. Hulchiy M, et al. Receptors for thyrotropin-releasing hormone, thyroid-stimulating hormone, and thyroid hormones in the macaque uterus: effects of long-term sex hormone treatment, 2012
  15. Ticconi C, et al. Pregnancy-promoting actions of HCG in human myometrium and fetal membranes, 2007
  16. Korevaar TI, et al. Hypothyroxinemia and TPO-antibody positivity are risk factors for premature delivery: the generation R study, 2013
  17. Taylor PN, et al. Controlled Antenatal Thyroid Study: Obstetric Outcomes. Thyroid Research — Meeting abstracts from the 64th British Thyroid Association Annual Meeting, 2017
  18. Torremante P, et al. Free thyroxine level in the high normal reference range prescribed for nonpregnant women may reduce the preterm delivery rate in multiparous, 2011
  19. Nazarpour S, et al. Effects of levothyroxine treatment on pregnancy outcomes in pregnant women with autoimmune thyroid disease, 2017
  20. Negro R, et al. Levothyroxine treatment in euthyroid pregnant women with autoimmune thyroid disease: effects on obstetrical complications, 2006
  21. Gruters A. Diagnostic Tests of Thyroid Function in Children and Adolescents, 2003

Photo: Freestocks/Unsplash. Design: VLM Health



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Dr. Vedrana Högqvist Tabor

Dr. Vedrana Högqvist Tabor

CEO @Boost_HealthApp|| TEDx speaker || Cancer hunter || Hashimoto’s patient|| Parentpreneur || Learning from own mistakes since 1977