hypothyroidism

The clinical syndrome of hypothyroidism is marked by inadequate thyroid hormone production, resulting in a decreased rate of cellular metabolism. It may be primary, in which the dysfunction pertains to the thyroid gland itself, or secondary, due to hypothalamic or pituitary dysfunction .

Epidemiology

Disproportionately affects females in a 10:1 ratio compared to males. Clinically symptomatic hypothyroidism is less common than subclinical hypothyroidism and becomes more common with increasing age. The most common cause depends on the patient population; in the United States autoimmune diseases, especially Hashimoto thyroiditis, causes the majority of cases, whereas in highly iodine-deficient populations (such as those who reside in Southeast Asia, especially populations further inland) iodine deficiency is the most common cause .

Clinical presentation

Hypothyroidism affects almost every organ system, with common presenting complaints including:

  • shortness of breath
  • muscle/joint pain
  • hoarseness
  • periorbital/peripheral (non-pitting) edema
  • weight gain
  • menstrual irregularity
  • cold intolerance
  • constipation

In cases of severe hypothyroidism, patients may present with a myxedema coma, which presents with reduced/altered mental state, hypothermia, severe constipation (including myxedematous megacolon), bradycardia, hypotension, hypoventilation, hypoglycemia, and gross myxedema .

ECG
  • sinus bradycardia
  • prolonged QT interval
  • flattening/inversion of T waves
  • interventricular conduction delays
    • especially right bundle branch block

Pathology

Etiology

Pathology of the thyroid gland itself may result in primary hypothyroidism, whereas pathology which affects the pituitary and hypothalamic production of thyroid-stimulating hormone (TSH) and/or thyroid releasing hormone (TRH) (respectively) is referred to as secondary hypothyroidism. Common etiological associations are as follows :

Radiographic features

Ultrasound

Sonographic appearance depends on underlying etiology, and may include:

  • abnormal size of the thyroid gland
  • alteration in thyroid echotexture
    • may be diffuse or nodular
  • abnormal color flow Doppler patterns

Treatment and prognosis

Management is with thyroid hormone replacement, typically with levothyroxine, a synthetic form of thyroxine, a.k.a. 'T4'.