Disfungsi

CHAPTER 19

Thyroid dysfunction Colin M. Dayan  •  Onyebuchi E. Okosieme  •  Peter Taylor

CHAPTER OUTLINE INTRODUCTION  373 NORMAL THYROID PHYSIOLOGY  374 The thyroid gland  374 Biological actions of thyroid hormones  374 Synthesis, storage and release of thyroid hormones  375 Iodine and thyroid hormone synthesis  376 Transport of thyroid hormones in blood  376 Free hormone hypothesis  376 Entry of thyroid hormone into tissues  377 Thyroid hormone deiodination and regulation of extrathyroidal T3 production  377 Catabolism of thyroid hormones  377 Nuclear action of thyroid hormones  377 Control of thyroid hormone synthesis and secretion  378 Extrathyroidal factors that may affect thyroid function  379

THE EVALUATION OF THYROID FUNCTION  382 Clinical evaluation of thyroid status  382 In vitro tests of thyroid activity and pituitary–thyroid status  383 Measurement of thyroid stimulating hormone  383 Free T4 and free T3 measurements  383 Methods for measuring free thyroid hormones  383 Validity of commercial methods for free hormone analysis  383 Nomenclature of free thyroid hormone assays  384 Total T4 and total T3  384 Selective use of thyroid function tests  384

INTRODUCTION Thyroid hormones are essential for normal growth, ­development and metabolism, and their production is tightly regulated through the hypothalamic–pituitary– thyroid axis. Thyroid disease is common, particularly in women, with a prevalence in the community of 3–5%. With the exception of iodine deficiency, which affects millions of people worldwide, diseases that directly

Interpreting results of thyroid function tests  385 Common situations in which TSH results may be misleading  385 Reference ranges and significant changes  385 Miscellaneous tests  386 Autoantibodies to thyroidal antigens  386 Imaging the thyroid  387

HYPERTHYROIDISM 388 Clinical features  388 Causes of hyperthyroidism  389 Hyperthyroidism or non-thyroidal illness?  394

HYPOTHYROIDISM  394 Clinical features  394 Causes of hypothyroidism  396 Treatment of hypothyroidism  397

THYROIDITIS  398 Thyroiditis producing hyperthyroidism  398 Hypothyroidism resulting from Hashimoto thyroiditis  399 Other forms of thyroiditis  399 Hypothyroidism and the postpartum period  399

NEOPLASIA  399 Diagnosis  399 Treatment  400 Tumour markers  400

SYNDROMES OF RESISTANCE TO THYROID HORMONES  400 SCREENING  401

affect the thyroid gland are the most common causes of thyroid dysfunction. Pituitary disease and the use of ­certain drugs that alter thyroid hormone synthesis or metabolism can also give rise to thyroid dysfunction. Any severe illness can produce abnormalities in the r­ esults of thyroid function tests that resolve as the patient’s i­llness improves. Once diagnosed, thyroid disease is usually easily treated, with an excellent long-term outcome for most patients. This chapter outlines thyroid physiology and the 373