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Table 24-1 Hormone Changes in Thyroid Disease
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Epidemiology

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(Ann Intern Med 2009;151:ITC6): 1–2% ♀; 0.1% ♂ (10:1 ratio); chronic thyroiditis ↑ risk of T1DM, Addison's, pernicious anemia, vitiligo, RA

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Pathophysiology

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Classification & Etiology

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(Ann Intern Med 2009;151:ITC6)

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  • Primary: failure of thyroid gland (99% of cases)
    • Hashimoto's disease: most common cause; chronic autoimmune thyroiditis; (+) TPO antibodies in >90%
    • Other causes: radioiodine ablation, head & neck radiation, medications
    • Subclinical: mild/moderate ↑ TSH, normal T4 & T3; 4–10% of population; 18% of elderly; 2–5% of these convert to overt hypothyroid per year; associated risks → HLP, CV, & neuropsychiatric effects
  • Secondary: pituitary or hypothalamus failure; causes: tumor, inflammatory conditions, infiltrative diseases, infections, pituitary surgery, pituitary radiation therapy, head trauma
  • Myxedema coma: life-threatening severe hypothyroidism induced by precipitating factor
    • TSH & FT4 not always significantly abnormal; most common in elderly with hypothyroidism

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Diagnosis & Evaluation (Hypothyroidism)

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  • Screening: not recommended in general population (Ann Intern Med 2004; 140:128); screen in pregnancy (J Clin Endocrinol Metab 2005;90:581)
  • Diagnosis (Ann Intern Med 2009;151:ITC6)
    • TSH: best diagnostic test for primary hypothyroid; ↑ TSH & T4 ↓ = overt hypothyroid; ↑ TSH & T4 normal = subclinical hypothyroid
  • Signs/symptoms: fatigue, weakness, lethargy, bradycardia, weight gain, impaired memory & learning, dry skin, cold intolerance, constipation, paresthesias, hoarseness, sleepiness, hair loss, sexual dysfunction, menstrual irregularity, depression (Ann Intern Med 2009;151:ITC6)
  • Physical exam: HTN, periorbital puffiness, thinning of lateral eyebrows, delayed relaxation phase of DTRs
  • Lab findings: macrocytic anemia, hyponatremia, hypercholesterolemia, ↑ creatine kinase
  • Myxedema coma: hypothermia, hypotension, bradycardia, seizures, stupor, coma, myxedematous skin changes, periorbital edema, distended abdomen & bladder

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Clinical Pearl 24-1

TSH may be slightly elevated in cases of acute non-thyroid illnesses (termed euthyroid sick syndrome); recheck TSH in 6–8wk (Ann Intern Med 2009;151:ITC6)

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Clinical Pearl 24-2

Glucocorticoids, dopamine, & octreotide can cause transient decreases in TSH (Ann Intern Med 2009;151:ITC6)

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Treatment & Follow-Up

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Pharmacologic Treatment

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(Ann Intern Med 2009;151:ITC6; Lancet 2004;363:793)

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  • Levothyroxine (LT4); treatment of choice
    • Starting dose: 1.6–1.7mcg/kg/d in young, healthy adults (∼125mcg in 70kg person); lean body mass better predictor than TBW; in obesity, dose based on IBW; titrate Q6–8wk
    • Elderly/known heart disease: 0.5mcg/kg/d (or 25–50mcg/d); ↑ by 12.5–25mcg Q6–8wk (full doses may lead to dysrhythmias, angina, or MI)
    • Goal: TSH 0.5–2.0mU/L
  • Liothyronine (LT3): not recommended; shorter half-life & faster onset ↑ thyrotoxicosis risk
  • No proven ...

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