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The thyroid gland produces two hormones derived from the amino acid tyrosine: triiodothyronine (T3) and tetraiodothyronine, or thyroxine (T4). These molecules contain three and four iodine atoms respectively. The thyroid secretes mainly T4, most of which the target cells convert to T3, as T3 has a greater affinity for the receptor in the cell nucleus. The thyroid plays a role in vertebrate development, maturation and homeostasis. Throhormones help maintain blood pressure, heart rate, muscle tone, digestion, and reporductive functions. T3 and T4 generally increase the rate of oxygen consumption and cellular metabolism. Excess hormones (hyperthyroidism) causes a high body temperature, excessive sweating, weight loss, irritability, and high blood pressure. Hypothyroidism can cause cretinism in infants and weight gain, lethargy, and intolerance to cold in adults.

Thyroid malfunction can cause a number of fertility problems, including menstrual irregularities and anovulation (lack of ovulation). The latter occurs through the upsetting of the body's normal hormone balance; excess or lack of thyroid hormones can reduce the levels of luteinising hormone (LH), which is needed to induce ovulation. Even if ovulation does take place, the luteal phase - the time between ovulation and menstruation - can be shortened. If the luteal phase is too short, the attachment of the fertilised egg cannot be sustained and menstruation occurs as normal, taking the egg with it.

Hypothyroidism can increase levels of prolactin, the hormone responsible for maintaining the production of breast milk after childbirth. Too much prolactin can prevent ovulation or irregular or absent menstruation. This could be caused by elevated levels of TRH (thyrotropin releasing hormone), which stimulates production of both prolactin and TSH (thyroid stimulating hormone). Hypothyroidism can also cause ovarian cysts which can impair ovulation.

Hyperthyroidism can cause miscarriages, premature births, and intrauterine growth retardation if undiagnosed or untreated. Autoimmune thyroid conditions cause the body to produce antithyroid antibodies, which have been shown to increase the chances of miscarriage. It is not always clear exactly how the thyroid affects fertility, but it is certain that it does.

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