Pregnancy and an underactive thyroid gland
BMJ Group Medical Reference
If you're pregnant and have an underactive thyroid gland, it's important to keep taking your treatment.
If you think you might have an underactive thyroid and are pregnant or thinking about getting pregnant, see your doctor. Not treating your underactive thyroid may harm your health or your baby's health. 
For example, you are at a higher risk of getting high blood pressure, bleeding, or having a stillborn baby if you don't get the right amount of thyroid hormones when you're pregnant. There's also a chance that your baby will be born with a low birth weight.
You'll have treatment with levothyroxine. This is a man-made version of the natural hormone thyroxine, the main hormone your body stops making when you have an underactive thyroid. It's safe to take levothyroxine during pregnancy. It won't harm your baby.
Even if you have a mildly underactive thyroid with no symptoms, it's still important to get treatment.
You'll need to have your levels of thyroid hormones checked at least every six weeks during your pregnancy. This is because your dose of levothyroxine may need to change. 
About 5 in 100 women get a mild form of hypothyroidism after they have their baby.  Most women get better on their own after a few months.
high blood pressure
Your blood pressure is considered to be high when it is above the accepted normal range. The usual limit for normal blood pressure is 140/90. If either the first (systolic) number is above 140 or the lower (diastolic) number is above 90, a person is considered to have high blood pressure. Doctors sometimes call high blood pressure 'hypertension'.
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