Undiagnosed and untreated thyroid disease can be a cause for infertility or recurrent miscarriage. The thyroid hormone is important for growth and metabolism , and because it regulates cellular function, abnormal thyroid function can affect your fertility.

Hypothyroidism -- an underactive thyroid -- is a frequent cause of infertility. If the thyroid in underactive, the hypothalamus and pituitary gland can sense this 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 , produced by the hypothalamus, 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 also 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 (thyroid-stimulating hormone ) is the most sensitive way to predict thyroid abnormality.

If it's abnormal, then you will need to do a complete thyroid panel , for testing your T3, T4 and TSH levels.

A normal TSH level is between 0.5 to 5.5 mIU/ml (with over 5.5 being considered hypothyroid, or underactive, and under 0.5 being hyperthyroid, or overactive.). Note, however, that there is an ongoing controversy among endocrinologists, and growing numbers of experts believe that the normal range needs to be revised, to 0.3 to 3.0. If you have no symptoms but your TSH is high, you have subclinical hypothyroidism.

Some infertility specialists believe that for optimal fertility, the TSH level should be 2 mIU/ml or less, and will treat patients with thyroxine in order to do so, though this is still controversial.

If you are found to have hypothyroidism, thyroid hormone replacement ( Eltroxine) is very effective in restoring fertility. It may take 1-2 months to normalize your thyroid hormone levels with medication ; and once you start medication, you must check your TSH levels to confirm they are in the normal range. You will usually need to continue the medicines for the rest of your life and recheck your thyroid function every 6 months or so.

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