Many patients with fatigue as a main complaint are eventually diagnosed as having an endocrine problem. However, an often-overlooked reason for fatigue in women who are menstruating is iron-deficiency. Iron is needed for thyroid hormone biosynthesis and action, including the conversion from the inactive thyroid hormone, T4, to the active hormone, T3. Women taking thyroid hormone are especially likely to benefit from treating iron deficiency. Mild iron-deficiency leads to low ferritin in blood tests BEFORE a drop in hemoglobin and hematocrit occurs. Since Dr. Friedman’s goal is early diagnosis of treatable diseases, he recommends measuring a ferritin level in all women who have fatigue.