Overview of primary hypothyroidism

Primary hypothyroidism refers to a disorder within the thyroid gland that reduces the production or secretion of thyroid hormones (thyroxine [T4] and triiodothyronine [T3]). The negative feedback response of serum thyroid hormone concentrations on thyroid stimulating hormone (TSH) results in an elevated serum TSH concentration.

Primary hypothyroidism can be either overt or subclinical:

  • Overt primary hypothyroidism is defined by an elevated serum TSH concentration and a low serum T4 concentration, and is usually symptomatic. It occurs in around 0.5% of the Australian population.
  • Subclinical primary hypothyroidism (also called mild thyroid failure) is defined by an elevated serum TSH concentration, but a serum T4 concentration within the normal range. It is often asymptomatic. Subclinical hypothyroidism occurs in around 5% of the Australian population.

Primary hypothyroidism is more common in:

  • women over 55 years
  • people with a history of Graves disease (remitted Graves disease can be followed by hypothyroidism)
  • people with an autoimmune disorder (eg type 1 diabetes, coeliac disease) or a condition that increases the risk of an autoimmune disorder (eg Down syndrome, Turner syndrome)
  • people taking some drugs (see Drugs that influence thyroid hormones).

Most cases of spontaneous primary hypothyroidism are due to autoimmune thyroiditis (Hashimoto thyroiditis). Thyroid peroxidase antibodies are almost always present in Hashimoto thyroiditis. They indicate an increased risk of progression of disease—a patient with subclinical hypothyroidism who has a positive thyroid peroxidase antibody is more likely to progress to overt disease than a patient a with negative thyroid peroxidase antibody.

Primary hypothyroidism is usually managed with thyroxine replacement therapy; see here for when to start therapy in adults and see here for dosing and principles of use of thyroxine replacement therapy in adults.

For management of children with hypothyroidism, see here, and for pregnant women with hypothyroidism, see here.