Hypothyroidism

Louis Brown, Griffin Bullock

Background

  • Elevated TSH and low FT4 (primary hypothyroidism)
    • Hashimoto’s (autoimmune) thyroiditis, iodine deficiency, drugs (amiodarone, dopamine antagonists, lithium), 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.
  • For subclinical hypothyroidism or goiter, measure serum TPO antibodies.

Management

  • Treatment required if ↓ T4, significantly ↑ TSH (>10), or symptoms with any lab abnormality. Treatment for TSH 4.5-10 & normal T4 remains controversial.
  • Titrate therapy to a normal TSH 0.5 to 4.5 mU/L (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 (>60) 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, zinc, copper, magnesium, multivitamins, PPIs interfere the most with absorption.
  • Advise at least 4 hours between levothyroxine and interfering medications.
    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 12.5 mcg to 25 mcg every 6 weeks based on TSH.
  • Do not adjust dose based on persistent symptoms alone.
  • 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.
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