Second and third trimester of pregnancy with Hashimoto’s and an underactive thyroid
How hypothyroidism affects mid and late-stage pregnancy
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
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
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 pediatrician 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
We’re aware there’s a lack of knowledge on the relationship between the thyroid/Hashimoto’s and pregnancy. Help us increase this knowledge by taking part in this survey.
Smith A, et al. Thyroid disorders in pregnancy and postpartum, 2017
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Women’s Health Committee; The Royal Australian and New Zealand College of Obstetricians and Gynaecologists. Testing for hypothyroidism during pregnancy with serum TSH, 2015
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Medici M, et al. Maternal early pregnancy and newborn thyroid hormone parameters: The generation R study, 2012
Wilson KL, et al. Subclinical thyroid disease and the incidence of hypertension in pregnancy, 2012
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Taylor PN, et al. Controlled Antenatal Thyroid Study: Obstetric Outcomes. Thyroid Research — Meeting abstracts from the 64th British Thyroid Association Annual Meeting 2017
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
Nazarpour S et al. Effects of levothyroxine treatment on pregnancy outcomes in pregnant women with autoimmune thyroid disease, 2017
Negro R, et al. Levothyroxine treatment in euthyroid pregnant women with autoimmune thyroid disease: effects on obstetrical complications, 2006
Gruters A. Diagnostic Tests of Thyroid Function in Children and Adolescents, 2003