Chapter 96: Thyroid Disorders

KEY CONCEPTS

• Thyrotoxicosis is most commonly caused by Graves’ disease, which is an autoimmune disorder in which thyroid-stimulating antibody (TSAb) directed against the thyrotropin receptor elicits the same biologic response as thyroid-stimulating hormone (TSH).

• Hyperthyroidism may be treated with antithyroid drugs such as methimazole (MMI) or propylthiouracil (PTU), radioactive iodine (RAI: sodium iodide-131 [¹³¹I]), or surgical removal of the thyroid gland; selection of the initial treatment approach is based on patient characteristics such as age, concurrent physiology (eg, pregnancy), comorbidities (eg, chronic obstructive lung disease), and convenience.

• MMI and PTU reduce the synthesis of thyroid hormones and are similar in efficacy, although their dosing ranges differ by 20-fold. Overall, PTU has a greater incidence of side effects. Agranulocytosis is a rare but severe adverse effect associated with both medications.

• Response to MMI and PTU is seen in 4 to 6 weeks and therefore β-blocker therapy may be concurrently initiated to reduce adrenergic symptoms. Maximal response is typically seen in 4 to 6 months; treatment usually continues for 1 to 2 years, and therapy is monitored by clinical signs and symptoms and by measuring the serum concentrations of TSH and free thyroxine (FT₄).

• Adjunctive therapy with β-blockers controls the adrenergic symptoms of thyrotoxicosis but does not correct the underlying disorder; iodine may also be used adjunctively in preparation for surgery and acutely for thyroid storm.

• Many patients choose to have ablative therapy with ¹³¹I rather than undergo repeated courses of MMI or PTU treatment; most patients receiving RAI eventually become hypothyroid and require thyroid hormone supplementation.

• Hypothyroidism is most often due to an autoimmune disorder known as Hashimoto’s thyroiditis.

• The drug of choice for replacement therapy in hypothyroidism is levothyroxine.

• Studies of combination therapy with levothyroxine and liothyronine have not shown reproducible benefits. This approach to the treatment of hypothyroidism requires further study.

• Monitoring of levothyroxine replacement therapy is achieved by observing clinical signs and symptoms and by measuring the serum TSH level. An elevated TSH indicates under-replacement; a suppressed TSH indicates over-replacement.

BEYOND THE BOOK

To get a basic understanding of hyperthyroidism and hypothyroidism, visit the Websites of National Institutes of Health and National Institute of Diabetes and Digestive and Kidney Diseases for a quick overview:

https://www.niddk.nih.gov/health-information/endocrine-diseases/hyperthyroidism

https://www.niddk.nih.gov/health-information/endocrine-diseases/hypothyroidism