Thyroid function in pregnancy

Thyroid function in pregnancy

Thyroid function in pregnancy

August 26, 2021

Undetected thyroid issues may impact the health of both mother and baby. Read on to learn more about thyroid function and pregnancy.

The thyroid, a butterfly-shaped gland located in front of the neck, is responsible for the production of the thyroid hormones thyroxine (T4) and triiodothyronine (T3). These hormones have a critical role in metabolism and growth. During the first few months of pregnancy, the unborn baby is unable to produce thyroid hormones, and is reliant on the mother, with only thyroxine (T4) being able to cross the placenta.

On becoming pregnant, the thyroid gland in women produces increased levels of thyroid hormones under the influence of the hormone human chorionic gonadotropin (hCG). Typically, thyroid hormones are produced under the instruction of a further hormone, thyroid stimulating hormone (TSH), which is produced by the pituitary gland in the brain. However hCG, which is produced by the placenta of pregnant women mimics the effect of TSH on the thyroid gland. When hCG levels rise, TSH levels will decrease slightly. The slight increase in thyroid hormones during early pregnancy is considered normal but can be linked to symptoms such as morning sickness and palpitations (when the heart beats faster than normal).

Hypothyroidism (underactive thyroid) during pregnancy, seen in about 1% of pregnant women, is associated with risks for both mother and baby. Often the diagnosis of underactive thyroid can be overlooked, as many of the signs and symptoms such as weight gain, fatigue and constipation, can be interpreted as “normal” during pregnancy. It is critical however to detect underactive thyroid promptly, even if the condition is only subclinical (when the levels of thyroid hormones are low but no symptoms are present), since the baby will not be able to produce any thyroid hormones independently during the first trimester. Low levels of thyroid hormones in the mother will result in low levels in the unborn baby, affecting brain development and thus cognitive ability. Untreated overt hypothyroidism during pregnancy also leads to increased risk of miscarriage, preeclampsia, premature delivery and low birth weight.

Melio's Pregnancy Health check includes:

  • Free Thyroxine (FT4): the most abundant thyroid hormone in the blood and the thyroid hormone that can be passed from mother to baby via the placenta. A FT4 test measures the amount of T4 that is not bound to protein and thereby freely available for use by cells.

  • Thyroid Stimulating Hormone (TSH): a hormone produced by the pituitary gland, and responsible for regulating the secretion of the thyroid hormone T4 and to a smaller extent also T3 from the thyroid gland.


The reference ranges for thyroid tests during pregnancy are different from that in non-pregnant women and are also different depending on the trimester. Other considerations such as recent infection or if you already have a known diagnosis of thyroid disease and are on treatment will affect your results. Thyroid test results can also be different depending on the laboratory and technique used. Doctors usually also take into account a number of factors when evaluating the results.

High TSH and normal FT4 levels are associated with:

  • Subclinical hypothyroidism (underactive thyroid)

High TSH and low FT4 levels are associated with:

  • Overt hypothyroidism (underactive thyroid)

Low TSH and high FT4 levels are associated with:

  • Hyperthyroidism (overactive thyroid)

Other Considerations

Results of thyroid function tests can be affected by temporary infections such as common colds causing both high and low values.

Taking high levels of biotin (vitamin B7) supplement can affect the readings of your thyroid function test. If you are regularly taking 5 mg (or 5000 mcg) of biotin supplement daily, or more, it is advisable to stop taking this 48 hours prior to the blood draw.

If you already have a diagnosed thyroid condition, you will need to adjust your medication dose during pregnancy. It is important that you discuss this with your midwife or GP.


2014 European Thyroid Association Guidelines for the Management of Subclinical Hypothyroidism in Pregnancy and in Children. John Lazarus et al. Eur Thyroid J. 2014 Jun; 3(2): 76–94.