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Thyroid problems during pregnancy

The thyroid gland at the neck produces thyroid hormones which regulate the metabolism in the body. Common disorders of the thyroid gland include excessive production of thyroid hormones or reduced production of thyroid hormones.

Hyperthyroidism ("Overactive thyroid gland")

This disorder may cause symptoms like heat intolerance, increased heart rate, palpitations, mood changes, vomiting and neck swelling. These symptoms are present in some pregnant women without an overactive thyroid gland too. More discriminatory features would include weight loss, tremor, and persistently increased heart rate (usually above 100 beats per minute).

95% of hyperthyroidism in pregnancy are due to an autoimmune disorder also known as Graves' disease. A blood test that shows both a raised free thyroxine and a reduced TSH (thyroid stimulating hormone) allows the diagnosis of hyperthyroidism to be made.

Untreated, hyperthyroidism increases the risk of miscarriage, fetal growth restriction (reduced growth of the fetus) and preterm labour. Rarely, the autoimmune antibodies that cause Graves' disease in the mother may also cause hyperthyroidism in 1% of her fetus or the newborn via the placental transfer of thyroid stimulating immunoglobulin (TSI). When the hyperthyroidism is well controlled with low doses of carbimazole (<15 mg/day) or PTU (< 150 mg/day), the outcome for the mother and fetus is generally good.

Hyperthyroidism in pregnancy is usually treated with carbimazole or PTU (propylthiouracil).

Hypothyroidism ("Underactive thyroid gland")

Hypothyroidism may cause symptoms like weight gain, lethargy, tiredness, hair loss, dry skin, constipation, carpal tunnel syndrome and water retention. These are symptoms present in some pregnant women without an underactive thyroid gland as well. More discriminatory features include cold intolerance and slow pulse rate (< 60 beats per minute).

Most cases are due to autoimmune destruction of the thyroid gland. Some may be due to treatment of hyperthyroidism (e.g. radioiodine, surgery, anti-thyroid medications). The commonest types are Hashimoto's thyroiditis and treated Graves' disease. A blood test that shows both a reduced free thyroxine and an increased TSH (thyroid stimulating hormone) allows the diagnosis of hypothyroidism to be made.

If the hypothyroidism is untreated, there is an increased risk of miscarriage, anaemia, low birthweight babies, and babies with neurodevelopmental delay especially if it is untreated and severe in the late first and early second trimesters of the pregnancy.

Treatment is generally with 100-200 microgram per day. Blood tests should be done every 4-6 weeks if there are any adjustments in thyroxine doses, and in 12 weeks if there are no adjustments.


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