The thyroid is the gland most responsible for your metabolic rate and, thus, your basal temperature.The quality and quantity of the hormones it secretes can mean the differance between living a 'normal' life and one that can be a living hell.

The normal basal body temperature range is between 97.7 and 98.2. Low basal temperatures, below 97.7, may reflect hypothyroidism; high basal temperatures, above 98.2, may be evidence of hyperthyroidism.

The function of the thyroid gland can be determined by measuring your basal
temperature and that can be done with something as simple as
a thermometer.

Menstruating women must take the basal temperature test for thyroid function only on the 1st, 2nd, 3rd or 4th day of menses(preferably beginning on the 2nd day).

Males, pre-pubertal girls, and post-menopausal or non-menstruating women may take basal temperatures any day of the month.

Women taking progesterone should not take it the day before and the days that the basal temperatures are taken.