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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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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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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 ...