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Importance of Thyroid Hormones in pregnancy


Thyroid physiology plays a major role in pregnancy. It has been reported that thyroid diseases occur in about 1%-2% of pregnant women and are more in women of children age, during pregnancy as well as post partum.

It is well know that thyroid diseases affect the outcome of pregnancy. Since the developing fetus synthesizes thyroid hormones only by the end of the first trimester, it depends on maternal thyroid hormones for organogenesis (organ creation) and central nervous system development as well as its general growth. Moreover, thyroid hormones are essential for the maintenance as well as successful completion of a normal pregnancy. Even after the delivery, thyroid hormones are essential during the postpartum period, playing a significant role in improving lactation.

Alternately, pregnancy may modify pre-existing thyroid dysfunction. However, the early detection of thyroid dysfunction and treatment of mother during pregnancy improves the outcome.

All these facts necessitate that the clinician be familiar with the thyroid physiology and management of thyroid diseases during pregnancy.


Physiology of Thyroid during Pregnancy

Now, let us discuss the normal changes in thyroid during pregnancy.

Thyroid hormone level increase during the first trimester of pregnancy and reach a plateau by midgestation until delivery. Similarly, free thyroid hormone levels  also increase during the first trimester and decrease to reach a plateau  in the second and third trimesters of pregnancy.

Excess of thyroid hormones leads to Hyperthyroidism and less of thyroid hormones leads to Hypothyroidism. Hypothyroidism is much more common than hyperthroidism.


Significance of Thyroid Hormone in Pregnancy

Maternal subclinical and thyroid dysfunctions demand attention. Maternal hyperthyroidism in past or present, carries risk since antibodies can cross the placenta. Autoantibodies to thyroid peroxidise and thyroglobulin have been reported to cause miscarriage.


Impact of Untreated Gestational Hypothyroidism

Untreated gestational hypothyroidism in mother can lead to hypertension (high blood pressure), preeclampsia, anemia (low hemoglobin), postpartum hermorrhage (excess bleeding), cardiac disorders, and spontaneous abortion.

The fetal complication of untreated maternal hypothyroidism include low birth weight, abnormal brain development, defects in brain structure, adverse effect  on cognitive function, as well as fetal death or stillbirth. On the other hand, in neonates, there are possible risks of deafness, neurological impairment, lowered IQ, and developmental delay.


Impact of Untreated Gestational Hyperthyroidism

The most common cause of maternal hyperthyroidism during pregnancy is Graves’ disease. It occurs in 1 in 1500 pregnant patients.

Hyperthyroidism in mother can lead to abortion, tachycardia (increased heart rate), sweating, and dyspnea (difficulty in breathing). Placental abruption, preterm delivery, preeclampsia, and thyroid storm are other related problems. Women with active Graves’ disease during pregnancy are at higher risk of developing very severe hyperthyroidism known as thyroid storm. Graves’ disease often improves during the third trimester of pregnancy and may worsen during the post partum period.

In fetus, it can cause neonatal thyrotoxicosis due to transplacental transport of TSH-Releasing Hormone stimulating autoantibody.


Diagnosis and Management Strategy for Thyroid Dysfunctions

General recommendations for hypothyroidism

An early detection is always recommended for hypothyroidism in pregnancy. Early detection of thyroid dysfunction during pregnancy is possible if the patient is suggested Thyroid Function Tests during her first prenatal visit or soon after the pregnancy is confirmed.

In a previously diagnosed hypothyroid woman on medication, adjustment of thyroxin dosage is necessary so that the level of TSH is adjusted to be 2.5mU/L before pregnancy. During pregnancy, the dosage of the drug should be maintained at a trimester-specific range. TSH should be monitored every 6-8 weeks.

If hypothyroidism is diagnosed during pregnancy, Thyroxin dosage needs to be adjusted to trimester-specific range and Thyroid Function Tests needs to be repeated in 30-40 days.

T4 replacement therapy is also recommended in case of subclinical hypothyroidism for a better obstetric outcome.