8- Hypothyroidism

Cards (23)

  • Hypothyroidism
    Underactivity of thyroid gland; may be primary (from the gland itself) or secondary to hypothalamic/pituitary disease (secondary/tertiary hypothyroidism)
  • Causes of hypothyroidism
    • Hashimoto's thyroiditis
    • Iatrogenic (thyroidectomy, radioactive iodine therapy, or external neck irradiation for head and neck malignancy)
    • De Quervain's thyroiditis (subacute viral thyroiditis)
    • Post-partum
    • Drug induced (carbimazole, lithium, amiodarone, interferon)
    • Iodine deficiency
    • Congenital hypothyroidism
  • Hashimoto's thyroiditis
    Autoimmune thyroiditis leading to hypothyroidism with goiter; cell and antibody mediated destruction of thyroid tissue; antibodies to thyroglobulin, thyroid peroxidase enzyme, or antibodies to block binding of TSH to receptor; associated with other autoimmune conditions (pernicious anemia, Addison's disease)
  • De Quervain's thyroiditis
    Subacute viral thyroiditis; follows a viral illness with a prodromal phase of fever & flu-like illness; painful, tender thyroid gland (may be enlarged); may cause transient hyperthyroidism due to leakage of hormones, followed by a euthyroid then a hypothyroid state; radioiodine uptake is diffusely low, High ESR; treatment is with NSAIDs or aspirin, and steroids in severe cases, with recovery of thyroid function within a year
  • Signs and symptoms of hypothyroidism
    • Tiredness, malaise, fatigue, depression
    • Weight gain
    • Cold intolerance
    • Bradycardia
    • Dry, brittle hair & dry skin, loss of lateral 1/3 of eyebrows
    • Periorbital edema (puffy eyes)
    • Goiter
    • Proximal myopathy
    • Slow-relaxing reflexes
    • Pitting edema
    • Carpal tunnel syndrome
    • Constipation
    • Menorrhagia (all bodily processes are slowed down except menstrual flow)
    • Diastolic hypertension
  • Periorbital & pitting edema
    Due to interstitial accumulation of glycosaminoglycans (hyaluronic acid) with associated extravascular water retention (osmotic effect)
  • Slow-relaxing reflexes, delayed relaxation phase: Woltman's sign

    Due to low myosin ATPase activity & low reaccumulation of calcium in sarcoplasmic reticulum
  • Conditions associated with slow-relaxing reflexes, delayed relaxation phase
    • Advanced age
    • Pregnancy
    • Sarcoidosis
    • Anorexia nervosa
    • DM
  • Bradycardia
    Low thyroid hormones decrease the heart rate and cardiac output due to the effect of thyroid hormones on alpha and beta adrenoreceptors (in hypothyroidism: density of α1-adrenoreceptors is increased while β-adrenoreceptors are reduced in vascular beds)
  • Diastolic hypertension
    Hypothyroidism causes a decrease in the release of endothelial-derived relaxing factor, which in turn promotes contraction of these cells, increasing PVR and reducing CO
  • Dry skin
    Vasoconstriction, water retention, thick skin, and reduced sweating
  • Investigations for thyroid disorders
    • TSH (high in primary & low in secondary/tertiary)
    • Serum free T4 level (will be LOW)
    • Thyroid antibodies and other organ-specific antibodies
    • Anemia (may be IDA from menorrhagia, autoimmune ex: pernicious, or anemia of chronic disease)
    • Hypercholesterolemia & hypertriglyceridemia, high LDL and low HDL
    • Hyponatremia (from high ADH & impaired free water clearance)
    • High CK, LDH, & AST
  • Management of hypothyroidism
    1. Lifelong Levothyroxine (starting dose: 1.6mcg/kg/day)
    2. Taken on an empty stomach, ideally 30-60 minutes before breakfast
    3. Serum thyroid hormone concentrations increase first and then TSH secretion begins to fall
    4. Goal of treatment is normalization of TSH level
    5. Symptoms may resolve after 2-3 weeks, but normal TSH levels are not achieved for at least 6 weeks
    6. TSH is assessed after 6 weeks on a steady dose, if still elevated increase dose by 25-50 and recheck in 6-8 weeks
    7. In conditions that cause an increase in serum binding proteins, the dosage of levothyroxine must be increased
  • Drugs that reduce thyroid hormone production
    • Lithium
    • Iodine-containing medications
    • Amiodarone
  • Drugs that reduce thyroid hormone absorption
    • Sucralfate
    • Ferrous sulfate (iron)
  • Drugs that increase metabolism of thyroxine
    • Rifampin
    • Phenobarbital
    • Carbamazepine
  • The normal range for TSH is 0.45 - 4.5 mU/L
  • In hypothyroidism, there will be an increase in TSH levels due to low free T3 & T4 levels
  • TSH is the most sensitive test to assess thyroid function
  • The normal range for FT4 is 9.0 - 19 pmol/L
  • Treatment of hypothyroidism involves replacement therapy with levothyroxine
  • The normal range for FT3 is 3.1 - 6.8 pmol/L
  • Free T3 and Free T4 are less sensitive tests than TSH but more specific