Endocrinology Book

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HypothyroidismAka: Myxedema

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  1. Epidemiology: Prevalence (US)
    1. Congenital: 1 in 4000 newborns
    2. Adults:
      1. Age over 65 years: 2-4% (especially older women)
      2. Overall: 0.5-1.0% (up to 18 cases per 1000 persons)
  2. Causes
    1. Hashimoto's Thyroiditis (Most common cause)
    2. Congenital Hypothyroidism (Cretinism)
    3. Thyroid ablation
      1. Graves' Disease Management
        1. Radioactive Iodine (I-131)
        2. Thyroidectomy
      2. Neck radiation in cancer treatment
        1. Hodgkin's Lymphoma
        2. Laryngeal Cancer
    4. Drugs
      1. See Medications Affecting Thyroid Function
    5. Secondary Hypothyroidism (only 5% of cases)
      1. Congenital Hypopituitarism
      2. Pituitary necrosis (Sheehan's Syndrome)
      3. Pituitary or Hypothalamic lesion
      4. HIV Infection on highly active Antiretrovirals
        1. Also associated with low CD4 Counts
        2. Beltran (2003) Clin Infect Dis 37:579
  3. Symptoms
    1. Generalized
      1. Fatigue or weakness (99%)
      2. Lethargy (91%)
      3. Cold intolerance (89%)
      4. Weight gain despite diminished food intake
      5. Edema
      6. Arthralgias
    2. Neuropsychiatric
      1. Diminished libido
      2. Headache
      3. Hoarseness
      4. Slow thinking
      5. Forgetfulness (66%)
      6. Slow speech (91%)
    3. Gastrointestinal
      1. Constipation (61%)
    4. Dermatologic
      1. Dry or coarse skin (97%)
      2. Decreased sweating (89%)
      3. Hair Loss (especially outer third of eyebrows)
      4. Broken nails
    5. Gynecologic
      1. Amenorrhea or Menorrhagia
  4. Signs
    1. General
      1. Round puffy face or other facial edema (79%)
      2. Periorbital or eyelid edema (90%)
      3. Large, thick Tongue or Macroglossia (82%)
      4. Non-pitting ankle edema
    2. Neuropsychiatric
      1. Slow speech
      2. Hoarse voice
      3. Hypokinesia
      4. Generalized Muscle Weakness
      5. Delayed relaxation of knee and ankle jerk reflex
    3. Dermatologic
      1. Cold, dry, thick Scaling skin
        1. Affects palms, soles, elbows and knees
        2. Skin may show yellow-orange discoloration
      2. Dry coarse brittle hair (76%)
      3. Dry, longitudinally ridged nails
    4. Cardiovascular
      1. Faint cardiac impulse
      2. Indistinct heart tones
      3. Cardiac enlargement
      4. Bradycardia
      5. Pericardial Effusion
      6. Variable effect on Blood Pressure
        1. Hypotension may be present
        2. Diastolic Hypertension
          1. Dernellis (2002) Am Heart J 143:718
    5. Gastrointestinal
      1. Ascites
  5. Pearls: TSH Screening Indications
    1. All elderly with depression
    2. All elderly entering long term care
  6. Labs
    1. See Thyroid Function Testing
      1. Serum TSH increased (most sensitive)
      2. Free T4 low and Free T3 low
        1. Primary Hypothyroidism
      3. Free T4 normal and Free T3 normal
        1. Subclinical Hypothyroidism
        2. Overt Hypothyroidism unlikely if Serum TSH 6-10
      4. Free T4 normal and Free T3 low
        1. Congenital absence of T4 to T3 converting enzyme
        2. Amiodarone can also reduce T4 to T3 conversion
    2. Other lab tests affected by Hypothyroidism
      1. Serum Cholesterol elevated
      2. Serum Triglyceride elevated
      3. Creatine Phosphokinase (CPK) elevated
      4. Lactate Dehydrogenase (LDH) elevated
      5. Complete Blood Count (CBC)
        1. Refractory Macrocytic Anemia
  7. Diagnostics: Electrocardiogram (EKG)
    1. Bradycardia
    2. Low amplitude QRS Complexes
    3. Flattened or inverted T Waves
  8. Imaging
    1. Retarded bone growth
  9. Complications
    1. Myxedema Coma
  10. Management
    1. See Thyroid Replacement (Levothyroxine)
    2. Elderly: Consider withdrawing Thyroid Replacement
      1. Indication: Elderly in Community and nursing-home
      2. May have been diagnosed prior to sensitive TSH
      3. Trial at decreased dose or off for 6 weeks
      4. Recheck TSH after trial
    3. Pregnancy: Avoidance of Hypothyroidism is critcal
      1. Check Thyroid Stimulating Hormone (TSH) early
      2. Consider increasing Synthroid dose by 30%
        1. Recheck TSH again in 4-6 weeks if dose increased
      3. Recheck TSH every trimester
  11. References
    1. Coll (2000) J Am Board Fam Pract 13:403
    2. Hueston (2001) Am Fam Physician 64(10):1717
    3. Singer (1995) JAMA 273(10):808

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