Last revised by Eve Kim on 30 Mar 2024

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 1.

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 3.

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 5.

  • sinus bradycardia

  • prolonged QT interval

  • flattening/inversion of T waves

  • interventricular conduction delays

    • especially right bundle branch block 2

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 thyrotropin releasing hormone (TRH) (respectively) is referred to as secondary hypothyroidism. Common etiological associations are as follows 1:

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

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

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