Anemia is the most common hematopoietic abnormality in endocrine disorders and may be the first manifestation of an endocrine disorder. Polycythemia/erythrocytosis is less common, but occurs in certain endocrine disorders. The pathophysiologic basis of the anemia is often multifactorial, but a direct influence of hormones on erythropoiesis in some instances may contribute to anemia. A decreased plasma volume in some of these disorders may mask the severity of anemia. It has been proposed that anemia in endocrine-deficiency states may be physiologic to adjust for decreased oxygen requirements. Some endocrine disorders are associated with an impaired response to the therapeutic use of erythropoietin.
Anemia is a well-recognized complication of thyroidectomy and other causes of hypothyroidism and may also occur in subclinical hypothyroidism.1 In a retrospective review, anemia defined as a hemoglobin less than 13 g/dL in men and less than 12 g/dL in women was present in 57 percent of patients with hypothyroidism.2 The anemia in hypothyroidism has been described variably as normocytic, macrocytic, or microcytic3 coexisting deficiencies of iron, vitamin B12, and folate may explain some of this heterogeneity. In a study of approximately 60 anemic patients with untreated primary hypothyroidism, 10 percent had a macrocytic anemia, all of whom had vitamin B12 deficiency, 43 percent had a microcytic anemia and iron deficiency, and the remainder had a normocytic anemia.4 However, even when these deficiencies have been excluded, some hypothyroid patients have a macrocytic anemia.5 In addition, although most hypothyroid patients have a significant reduction in their red cell mass, anemia is not always evident from hemoglobin and hematocrit values owing to a concomitant reduction of plasma volume.6,7
Hypothyroidism may contribute to the development of iron deficiency (Chap. 43) due to associated menorrhagia, although this association is less common than previously thought.8 Because thyroid hormone may augment iron absorption,9,10 iron deficiency in hypothyroidism may also be caused by impaired iron absorption, either directly from a lack of thyroid hormone or an associated achlorhydria.11,12 Conversely, iron deficiency impairs thyroid hormone synthesis by reducing the activity of heme-dependent thyroid peroxidase.13 In patients with coexisting iron-deficiency anemia and subclinical hypothyroidism, the anemia often does not adequately respond to oral iron therapy. Combined treatment with oral iron and levothyroxine results in superior improvement in hemoglobin and ferritin levels compared with levothyroxine alone in these patients.14,15
Although the macrocytosis seen in hypothyroid patients may be due to deficiencies of vitamin B124,5 or folate16 (Chap. 41), hypothyroidism also causes macrocytosis that resolves with thyroxine treatment.5 The mean corpuscular volume of hypothyroid patients with low vitamin B12 levels is similar to those with uncomplicated hypothyroidism, so this is not a sensitive means of identifying patients ...