![]() Replacement with levothyroxine is indicated for patients who have symptomatic hypothyroidism with a serum TSH > 10 µIU/mL or in patients with goiter and positive antithyroid peroxidase antibodies. Primary hypothyroidism may be overt (clinical) or mild (subclinical). Central hypothyroidism is associated with low free T4 and inappropriately normal or low TSH levels. Primary hypothyroidism is characterized by a low free T4 level and elevated TSH. Administration of glucocorticoids, dopamine, and dobutamine may interfere with accurate testing. Normal serum thyrotropin levels range from 0.4–4.5 mU/L. The diagnosis of hypothyroidism is confirmed by measurement of serum TSH and free thyroxine (T4). Elevated levels of T4 and T3 inhibit further release of TSH from the pituitary, completing the feedback loop. T3 is four times more potent than T4, but it is secreted in very small quantities by the thyroid. The majority of T4 is bound to proteins, with approximately 0.3% free in the circulation (free T4). The normal thyroid gland produces about 80% T4 and about 20% T3. All cells in the body depend upon thyroid hormones for regulation of their metabolism. A euthyroid state (normal thyroid function) is maintained by a feedback loop between the anterior pituitary gland, which produces thyroid-stimulating hormone (thyrotropin TSH) in response to thyrotropin-releasing hormones produced by the thyroid gland. ![]() The function of the thyroid gland is to take iodine, found in many foods, and convert it into thyrotropin-releasing hormones thyroxine (T4) and triiodothyronine (T3). A review of hypothyroidism, including presenting signs and symptoms, indications for treatment, and implications for the patient with cancer will assist in effective management of this very treatable disorder. The signs and symptoms of hypothyroidism may be attributed to treatment with these agents, and unnecessary dose modifications or discontinuation of potentially beneficial therapy may result. While the incidence with use of standard chemotherapeutic agents is uncommon, thyroid dysfunction from neck irradiation, immune therapy (interleukin-2, interferon), immunomodulatory agents (thalidomide, lenalidomide ), radioimmunoconjugates (131I-tositumomab ) and small-molecule inhibitors (sunitinib, sorafenib ) may result in either hyperthyroidism or hypothyroidism ( Table 2). Thyroid dysfunction as a result of cancer treatment is an often overlooked clinical problem. The clinical manifestations range from asymptomatic individuals to severe symptoms including congestive heart failure, adrenal insufficiency, psychosis, and coma (Table 1). The physiological effects of hypothyroidism are widespread and include cardiovascular, renal, gastrointestinal, neurological, and endocrine abnormalities. ![]()
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