Thyroid Disease in Pregnancy
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Direct answer
Thyroid disease in pregnancy is managed with trimester-specific TSH ranges, because normal pregnancy physiology lowers TSH in the first trimester. Women on levothyroxine usually need a higher dose once pregnancy is confirmed, and propylthiouracil is preferred for Graves disease in the first trimester, with carbimazole used later in pregnancy.
What you must remember
- Trimester ranges: TSH reference limits differ by trimester, and many guidelines advise using trimester-specific ranges rather than the non-pregnant range.
- Levothyroxine dose: requirement commonly rises by roughly 20 to 30 per cent in hypothyroid women, so the dose is increased as soon as pregnancy is confirmed.
- Graves disease: TSH receptor antibodies cross the placenta and can cause fetal or neonatal thyrotoxicosis, so they are measured in pregnancy.
- Antithyroid drug choice: propylthiouracil is preferred in the first trimester because carbimazole and methimazole are linked to embryopathy such as aplasia cutis and oesophageal or choanal atresia.
- Radioiodine: contraindicated in pregnancy, since iodine crosses the placenta and can ablate the fetal thyroid.
- Gestational transient thyrotoxicosis: hCG-driven TSH suppression in early pregnancy, peaking around 10 to 12 weeks, usually needs no antithyroid drug.
- Target control: antithyroid drugs are kept at the lowest effective dose, with free T4 aimed at the upper normal range.
Trimester-specific management
Pregnancy changes both the gland and the drug targets, so the same TSH result means different things at different gestations. The table shows how the common scenarios are handled.
| Situation | Preferred approach | Key point |
|---|---|---|
| Hypothyroid on levothyroxine | Increase dose at confirmation of pregnancy | Aim for TSH in the lower half of the trimester range |
| Graves disease, first trimester | Propylthiouracil at the lowest effective dose | Carbimazole is avoided because of embryopathy risk |
| Graves disease, second and third trimester | Switch to carbimazole | Keep free T4 in the upper normal range |
| Gestational transient thyrotoxicosis | Observe, often without a drug | hCG effect that settles as hCG falls |
| Thyroidectomy needed | Second trimester, once controlled | Radioiodine is never used in pregnancy |
Dosing follows the trimester rather than the non-pregnant reference range, and levothyroxine doses are adjusted on repeat TSH measurements. Antithyroid drugs are kept as low as possible, because both under- and over-treatment carry fetal risk. Close shared care between the endocrinologist and the obstetrician keeps the regimen safe through delivery.
Where the exam tries to catch you
Examiners test the trimester logic, the drug choice and the diagnosis of transient hCG effects.
- Trap: Using the non-pregnant TSH range to judge treatment. Correct: Trimester-specific ranges apply, since first-trimester TSH falls physiologically.
- Trap: Choosing carbimazole for first-trimester Graves disease. Correct: Propylthiouracil is preferred in the first trimester, then switched to carbimazole in the second trimester.
- Trap: Treating every early thyrotoxicosis as Graves disease. Correct: High hCG in gestational thyrotoxicosis suppresses TSH, and TRAb testing and the clinical course help separate the two.
Frequently asked questions
Why does levothyroxine dose increase in early pregnancy?
Levothyroxine requirements rise in pregnancy because of higher thyroxine-binding globulin levels and fetal demand, so the dose is commonly increased by about 20 to 30 per cent once pregnancy is confirmed. TSH is then rechecked at regular intervals through the first half of pregnancy.
Why is propylthiouracil preferred in the first trimester?
Propylthiouracil is preferred in the first trimester of Graves disease because carbimazole and methimazole are associated with embryopathy such as aplasia cutis and oesophageal or choanal atresia. It is then usually switched to carbimazole in the second trimester, because propylthiouracil carries a higher risk of hepatotoxicity.
What is gestational transient thyrotoxicosis?
Gestational transient thyrotoxicosis is a self-limiting thyrotoxicosis in early pregnancy, caused by high human chorionic gonadotropin stimulating the TSH receptor. It peaks around 10 to 12 weeks and typically needs no antithyroid drug, with TSH receptor antibodies helping distinguish it from Graves disease.
Can radioactive iodine be used to treat Graves disease in pregnancy?
Radioactive iodine is contraindicated in pregnancy because iodine crosses the placenta and can destroy the fetal thyroid gland. When surgery is needed in pregnancy, it is usually performed in the second trimester after antithyroid drugs have controlled the disease.