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Hypothyroidism

Griffin Bullock

Background

  • Elevated TSH and low FT4 (primary hypothyroidism)
    • Hashimoto’s (autoimmune) thyroiditis, iodine deficiency, drugs (amiodarone, dopamine antagonists), adrenal insufficiency, thyroid hormone resistance (genetic), non-thyroidal illness (recovery phase), post-surgery or ablation for hyperthyroidism
  • Elevated TSH and normal FT4: subclinical hypothyroidism
  • Low-Normal TSH, low FT4: central hypothyroidism, sick euthyroid

Presentation

  • Often non-specific and vague: fatigue, cold intolerance, weight gain, constipation, dry skin, myalgia, edema, menstrual irregularities, depression, mental dysfunction
  • Goiter, bradycardia, diastolic hypertension, delayed relaxation following reflex testing
  • Lab abnormalities: microcytic anemia, hypercholesterolemia, hyponatremia, elevated CK

Evaluation

  • TSH: If elevated repeat TSH and obtain T4
  • Lipid panel, CBC, BMP

Management

  • Treatment required if ↓ T4, significantly ↑ TSH (>10), or symptoms with any lab abnormality
  • Titrate therapy to a normal TSH (unless central hypothyroidism, then target free T4 levels)
  • Observation of asymptomatic pts with subclinical hypothyroidism (normal T4, mild ↑ TSH)
  • Treatment is with formulation of T4 (full replacement is approximately 1.6 mcg/kg/day)
  • Initial Dose:
    • Young/healthy pts: full anticipated dose
    • Older pts or pts with CAD, atrial fibrillation: 25-50 mcg daily
  • Increased doses required for: pregnancy, estrogen therapy, weight gain, PPI therapy, GI disorders (↓ absorption), ferrous sulfate therapy
  • Risks of overtreatment: cardiac effects, increase risk of osteoporosis

Additional information

  • Pts should take Levothyroxine alone, 1h prior to eating to ensure appropriate absorption. Calcium, iron, PPIs interfere the most with absorption.
  • Of note, missed doses can be taken along with the next dose.
  • Symptoms improve in 2-3 weeks. TSH steady state requires 6 weeks.
  • Dose can be titrated every 6 weeks based on TSH.
  • Pregnancy: Pregnancy causes lab changes due to differing levels thyroid binding globulin. Use tables based on trimester to interpret values.
    • TPO antibody testing should be conducted, as if abnormal this affects risk of complications
    • Hypothyroid pts are at increased risk for preeclampsia, placental abruption, preterm labor/delivery
    • Refer to endocrine for close monitoring and adjustment to avoid fetal complications