Hypothyroidism

• definition: decreased production r action of thyroid hormone and its effects onteh body

• myxedema: severe form of hypothyroidism, as in myxedema coma. a non-pitting edema

• cretinism: severe hyperthyroidism in children, accompanied by arrested physical and mental development

• prevalence

  • 2% adult women

  • 0.1 to 0.2% of adult men

  • subclinical hypothyroidism: 0.5%

  • neonatal hypothyroidism: 1 in 3500 newborns

Major causes of hypothyroidism

• primary hypothyroidism: 98% of all cases; thyroid failure, decreased production of T4/T3 by thyroid gland

• secondary hypothyroidism: 1% of all cases; pituitary failure and decreased production of TSH

• other causes: < 1%; thyroid hormone resistance, consumptive (increased metabolism of T4/T3)

Etiology of hypothyroidism

• primary hypothyroidism: TSH elevated

  • destruction of functional thyroid tissue

    • autoimmune

      • Hashimoto thyroiditis

      • Graves disease (end-stage)

    • iatrogenic

      • radioiodine

      • surgery

    • infiltrative

      • Riedel thyroiditis

      • sarcoidosis, scleroderma

      • lymphoma

  • decreased hormone synthesis

    • decreased iodine intake

• secondary hypothyroidism—TSH not elevated

  • hypopituitarism

  • selective TSH deficiency

  • hypothalamic: TRH↓, termed tertiary hypothyroidism

• resistance to thyroid hormone

Specific features of hypothyroidism

• permanent: Hashimoto

• temporary: subacute or post-partum thyroiditis

• goitrous hypothyroidism: enlarged thyroid gland, Hashimoto thyroiditis

• nongoitrous hypothyroidism

• mild form: subclinical hypothyroidism

• acquired: Hashimoto; RAI, surgery

• congenital: peroxidase enzyme deficiency

Hypothyroidism symptoms: opposite those of hyperthyroidism

• mental: tired, apathetic, depressed

• nutritional: weight gain, fluid retention

• metabolism: decreased metabolic rate

• temperature: feeling cold

• kinetics: hypoactive, drowsy, lethargic

• GI: hypomotility and constipation

• cardiac: low-output heart failure

• muscular: weakness

• menstrual: menorrhagia, infertility

• ectodermal: dry skin

• eyes: periorbital swelling

Hypothyroidism: signs

• delayed ankle reflexes

• non-pitting edema (myxedema)

• carpal tunnel syndrome

• hypertension

• hypercholesterolemia

Hypothyroidism: diagnosis

• Symptoms and signs: tiredness, cold intolerance

  • delayed reflexes

  • decreased heart rate

• Thyroid function tests

  • TSH↑

  • free T4↓

  • thyroid peroxidase antibody, as in Hashimoto disease↑

Hypothyroidism lab values

• TSH↑

• FT4 below normal

Treatment of hypothyroidism

• T4 replacement: 0.1 to 0.15 mg (on average, 0.125 mg) thyroxin

• in patients with coronary artery disease, start at 0.025 mg and increase dose every 2-4 weeks

• T4 plus T3 replacement

• hypothyroidism during early pregnancy leads to lower baby IQ score

• congenital hypothyroidism: early T4 treatment for normal IQ

Important points about hypothyroidism

• Causes: primary hypothyroidism

  • thyroid destruction

    • iatrogenic

    • autoimmune (Hashimoto)

  • secondary hypothyroidism: TSH no t elevated

    • hypopituitarism

• Treatment: normalize T3, T4

Category: Endocrinology Notes