A low basal body temperature is typically a reading below about 96°F (35.6°C). The temperature should be taken while at rest, just after waking up, for it to be accurate. The result indicates the person's metabolic rate, so a low body temperature may signal hypothyroidism, which often results in side effects that include dry skin, depression and fatigue. For most women, though, this is considered normal for at least part of the month, because the body tends to have a lower temperature before ovulation as part of the biphasic pattern. This tends to occur during menstruation — in which case the typical effects of low temperature could be said to include vaginal bleeding, fatigue and mood swings — though the low temperature is more an effect of hormonal changes than a condition that causes other effects.
Some people keep track of their basal body temperature regularly, typically by using a thermometer each morning before getting out of bed. Taking the temperature at the same time each day tends to get the most accurate results, and most medical professionals advise that patients record the temperature for at least three days in a row to notice a pattern. This is because a random low temperature may not mean much, or it may be a result of a particularly cold night, but temperatures that are constantly lower than the average may signal a health problem. For example, since the metabolic rate is determined by the number of hormones the thyroid gland releases, low basal body temperature can indicate hypothyroidism.
An underactive thyroid gland causes a number of symptoms, but many people go undiagnosed because the effects are often attributed to other issues. For instance, many patients complain of headaches, dry skin, depression, fatigue, weight gain and sensitivity to the cold. Women with hypothyroidism also may notice irregular menstrual cycles. If this condition is left untreated for years, patients may suffer from slowed speech, thick skin and the inability to taste or smell as effectively as usual.
For many women, though, low basal body temperature is normal as long as it is only for about half the month. This is because the menstrual cycle uses a biphasic pattern in which the body's temperature is low for the first two weeks of the cycle and higher during the second half. A high level of estrogen is usually the culprit before ovulation, which is why the temperature increases just after the body ovulates, when progesterone takes over. Therefore, low temperature may be accompanied by both vaginal bleeding and symptoms of premenstrual syndrome (PMS), such as fatigue, mood swings and bloating.