Thyroid disease in pregnancy

Thyroid disease in pregnancy

Table of content

 

A)Hyperthyroidism

1. Incidence

2. Clinical features

3. Diagnosis

4. Effect of pregnancy on thyrotoxicosis

5. Effect of thyrotoxicosis on pregnancy

6. Management

 

B)Hypothyroidism

1. Incidence

2. Causes

3. Diagnosis

4. Clinical features

5. Effect of pregnancy on hypothyroidism

6. Effect of hypothyroidism on pregnancy

7. Management

References

Hyperthyroidism

Incidence

• Thyrotoxicosis complicates about 1 in 500 pregnancies
• Most common cause is Graves’ disease (95%)- (This is an autoimmune disease characterised by production of TSH receptors stimulating antibodies)
• Rarely in women of child bearing age hyperthyroidism may be due toxic multi-nodular goitre, toxic adenoma, subacute thyroiditis, acute (Viral) thyroiditis, Iodine, amiodarone or Lithium therapy
• Almost 50% of women have a positive family history of autoimmune thyroid disease
• Most cases are already diagnosed before pregnancy

Clinical features

• Heat intolerance, tachycardia, palpitations, palmar erythema, emotional liability, vomiting and goitre
• Most discriminatory features-Weight loss, tremor, a persistent tachycardia, lid lag, and exophthalmos
• Thyroid associated ophthalmopathy is present in up to 50% of patients with Graves disease
• If thyrotoxicosis occurs for the first time in pregnancy it usually presents late in the first or early in the second trimester

 

Diagnosis

• Raised free T4 or free T3
• Suppressed TSH – (This may be a feature of early pregnancy)
• Differentiation from Hyperemesis Gravidarum may be difficult

 

Effect of pregnancy on thyrotoxicosis

• Improves in pregnancy especially in the 2^nd and 3^rd trimesters
• Deterioration may occur in the first trimester, possibly related to hCG production.
• Pregnancy is a state of relative immunodefiency but with return of normal immunity in the puerperium it is also likely to deteriorate
• Pregnancy has no effect on Graves’ ophthalmopathy

 

Effect of thyrotoxicosis on pregnancy

• If severe and untreated, it is associated with inhibition of ovulation and infertility
• Increased rate of miscarriage, fetal growth restriction, preterm labour and perinatal mortality if untreated
• Thyroid stimulating antibodies may cause fetal or neonatal thyrotoxicosis
• Thyrotoxicosis may lead to sinus tachycardia, supraventricular tachycardia or atrial fibrillation.
• Thyroid crisis (storm) and heart failure may develop particularly at the time of delivery if poorly controlled
• There is usually good maternal and fetal outcome if the condition is well controlled with anti-thyroid drugs
• Rarely tracheal obstruction or dysphagia may be caused by retrosternal extension of goitre. This may lead to difficult intubation

 

Management

• Refer to Endocrinologist in early trimester for a care plan in pregnancy
• Anaesthetic review in early pregnancy
• Switch from Carbimazole to propylthiouracil before or in early pregnancy to avoid the unlikely risk of aplasia cutis (patches of absent skin affecting the scalp)

 

Treatment options for thyrotoxicosis

• Carbimazole (15-40mg) and Propylthiouracil PTU (150-400mg)
• PTU is usually avoided due to rare complication of liver failure
• Both Carbimazole and PTU cross the placenta, PTU less than Carbimazole
• Both drugs can cause fetal hypothyroidism and goitre if given to mum in high doses
• Carbimazole and methimazole may rarely cause aplasia cutis (patches of absent skin affecting the scalp) when used in the first trimester
• Control thyrotoxicosis as rapidly as possible
• Aim for woman to be clinical euthyroid with a free T4 in the upper end of the normal range for pregnancy
• Newly diagnosed thyrotoxicosis should be treated aggressively with high dose Carbimazole 45mg-60mg or PTU 450mg -600mg daily for 4-6weeks.
• Use beta blockers for tachycardia, tremors and sweating
• Beta blockers are discontinued once the anti-thyroid take effect and there is clinical improvement, usually within 3 weeks
• Radioiodine therapy is contraindicated in pregnancy and breastfeeding
• Avoid pregnancy for at least 4months after treatment with radioiodine in view of theoretical risk of chromosomal damage and genetic abnormalities

 

Surgery

• Thyroidectomy is reserved for those with dysphagia or stridor related to a large goitre, suspected or confirmed carcinoma and those with allergies to all antithyroid drugs.
• 25-50% will become hypothyroid following thyroid surgery, so follow up and treatment with replacement therapy may be required
• 1-2% incidence of hypocalcemia due to removal of parathyroid gland during thyroid surgery

 

Side effect of antithyroid drugs

• Drug rash or Urticaria 1-5%-switch to a different preparation
• Neutropenia/agranulocytosis occurs rarely-women should report any signs of sore throat or infection
• Liver impairment occurs rarely with Propylthiouracil (1in10000)

 

Antenatal care

• Do thyroid function test monthly
• Offer serial growth scan
• Refer for anaesthetic review
• Refer to endocrinologist for a robust care plan both in the antenatal and postnatal period
• Almost 30% can stop all medications in the last weeks of pregnancy

Postnatal care

• Graves’ disease can relapse postnatally as maternal antibody levels rises postpartum
• Retest all previously hyperthyroid women 2-4months after delivery
• In those who have stopped medication, it is often necessary to reintroduce it in the postnatal period
• Doses of PTU at or below150mg/day and Carbimazole 15mg /day are unlikely to cause problems in the fetus. It is safe to breastfeed at these doses
• Thyroid function test should be checked in umbilical cord blood and regular interval in the neonate if mum is taking high doses of antithyroid drugs

 

---

 

Hypothyroidism

Incidence

• Complicates about 1% of pregnancies
• Most cases are diagnosed before pregnancy with woman already on replacement therapy

 

Causes

• Commonest cause is due to autoimmune destruction of thyroid gland associated with microsomal autoantibodies
• This may be due to Hashimoto’s thyroiditis and treated Graves’ disease
• It may be iatrogenic following radioiodine therapy, thyroidectomy or related to drugs (Amiodarone, Lithium, iodine or anti-thyroid drugs)
• It is associated with autoimmune diseases like diabetes, vitiligo and autoimmune diseases

Diagnosis

• Made by a low T4, TSH is also raised
• Raised TSH in isolation is not diagnostic
• Note that free T4 are normally lower in the 2^nd and 3^rd trimester hence TSH levels is most useful

Clinical features

• Weight gain, lethargy and tiredness, hair loss, dry skin, constipation, carpal tunnel syndrome, fluid retention and goitre
• Most discriminatory features in pregnancy are cold intolerance and delayed ankle reflexes

Effect of Pregnancy on hypothyroidism

• Pregnancy probably has no effect on hypothyroidism
• About 25% of women require an increase in their dose of thyroxine
• The most common reason for increasing Levothyroxine is an inadequate pre-pregnancy dose

Effect of hypothyroidism on pregnancy

• Association with anovulatory infertility if untreated
• Severe or untreated hypothyroidism is associated with increased risk of miscarriage, fetal loss, pre-eclampsia and low birth weight
• Fetus requires adequate T4 before the end of 12weeks -Inadequate replacement may lead to reduce IQ in the offspring

 

Management

Preconception

• Check thyroid function in women planning a pregnancy to ensure adequate replacement prior to conception
• Reassure women that fetus is not at risk of thyrotoxicosis as only small amount of thyroxine crosses the placenta

 

Antenatal care

• Check thyroid function in early pregnancy and replacement should be commence immediately if required
• Check thyroid function every trimester
• Following adjustment in dose of levothyroxine, thyroid function should be checked after 4-6weeks
• Isolated raised TSH is common in the 1^st trimester and thyroxine doses should be increased if there low free T4 or raised TSH is persistent despite normal free T4 as this could be a case of subclinical hypothyroidism
• In newly diagnosed patient replacement with levothyroxine 100ug should be done immediately if there is no history of cardiovascular disease. With history of heart disease replacement should be done at a lower dose

 

Postnatal care

• Check thyroid function in the puerperium in women who had dose adjustment in pregnancy to ensure they are not rendered hyperthyroid.

 

References

1. Handbook of Obstetric medicine 5^th edition Catherine Nelson-Piercy
2. Oxford handbook of Obstetrics and gynaecology