The more common of the two main thyroid disorders is hyperthyroidism, the over-production of thyroid hormones. This can happen over either a short or long period of time, and it can affect many different functions of the body because of the thyroid's influence over metabolism [source: PubMed Health].
When the thyroid produces too much hormone, it can send the body into a state of over-activity, including high blood pressure, an increased appetite, and intolerance to heat and frequent sweating, as well as other side effects. When it comes to a woman's ovulation cycle, hyperthyroidism can cause irregular or even a complete lack of menstrual cycles [source: PubMed Health].
Most instances of hyperthyroidism are cause by Graves' disease, an autoimmune disorder that directly affects the thyroid. It is often treated by correcting the body's output of thyroid hormones [source: PubMed Health].
It is not impossible for a woman to get pregnant if she shows signs of hyperthyroidism. But, if she does have hyperthyroidism, it has to be treated and monitored by a doctor to protect both her and the baby during pregnancy.
Left unchecked, hyperthyroidism can lead to complications like preeclampsia, a spike in blood pressure late into the pregnancy, as well as premature birth, a low birth weight or even miscarriage [source: NEMDIS]. But these are only symptoms of severe hyperthyroidism. For women who have a mild case of the condition, treatment is not generally needed during pregnancy [source: NEMDIS].