Exactly how can a thyroid gland problem affect a woman's fertility? We have been trying to conceive for over a year. My menses are at 28 days, but the bleeding lasts for four days and stops, then three days later returns and goes another three days, sometimes light and sometimes very heavy. I had some tests, and my doctor told me about my thyroid. This could explain why I'm so tired all the time. Can it also affect my not being pregnant yet?

B.L.

Thyroid hormone is important for growth and metabolism. Symptoms of an underactive thyroid can include weight gain, constipation, dry skin, brittle nails, hair loss and cold intolerance. An overactive thyroid can cause diarrhea, weight loss, and palpitations.

Hypothyroidism -- an underactive thyroid -- is a frequent cause of infertility. The hypothalamus and pituitary gland can sense an underactive thyroid gland and try to kick things back to normal by increasing levels of the hormones TRH (thyroid-releasing hormone) and TSH (thyroid-stimulating hormone) in your body. TRH prompts the pituitary to release TSH, which in turn stimulates the thyroid to do its job. However, TRH also prompts the pituitary to release more of the hormone prolactin. Elevations of prolactin can interfere with ovulation by suppressing release of the hormones LH and FSH, which stimulate the ovary. Low levels of thyroid hormone can also interfere with the rate at which your body metabolizes sex hormones, which can cause ovulatory disorders.

Excess thyroid hormone -- resulting either from an overactive thyroid gland or from taking too much thyroid hormone -- can also cause problems. Thyroid hormone can block the effect of estrogen at various points in the body. That means the estrogen will not be as effective at getting its job done. This can make the endometrium (uterine lining) unstable and cause abnormal uterine bleeding.

A simple blood test for TSH or thyroid-stimulating hormone is the most sensitive way to predict thyroid abnormality. If you are found to have an underactive thyroid gland, thyroid hormone replacement can be effective at restoring fertility. It make take two to three months to regulate your thyroid hormone levels with medication; this should be done before you try to get pregnant.