An inadequate output of thyroid hormones by the thyroid gland

Causes

• Hashimoto’s thyroiditis:
  • Most common cause in the developed world.
  • Causes by autoimmune inflammation of the thyroid gland.
  • Ix - antithyroid peroxidase (anti-TPO) antibodies and antithyroglobulin antibodies.
  • Initially causes a goitre, after which there is atrophy of the thyroid gland
• Atrophic (autoimmune thyroiditis)
  • Anti-thyroid antibodies cause lymphoid infiltration of the gland and eventually atrophy and fibrosis
• Iodine deficiency:
  • Most common cause in the developing world.
  • Iodine is added to foods such as table salt to prevent iodine deficiency
• Secondary to Treatment of Hyperthyroidism:
  • Carbimazole
  • Propylthiouracil
  • Radioactive iodine
  • Thyroid surgery

Medications

• Lithium inhibits the production of thyroid hormones in the thyroid gland and can cause a goitre and hypothyroidism.
• Amiodarone interferes with thyroid hormone production and metabolism, usually causing hypothyroidism but it can also cause thyrotoxicosis.
  • It decreases T4 to T3 conversion

Central Causes (Secondary Hypothyroidism)

This is where the pituitary gland is failing to produce enough TSH. This is often associated with a lack of other pituitary hormones such as ACTH. This is called hypopituitarism and has many causes:

• Tumours
• Infection
• Vascular (e.g. Sheehan Syndrome)
• Radiation

Severe hypothyroidism leading to severe Sx and reduced levels of consciousness = myxoedema coma