Hypothyroidism is the term used to describe an inadequate output of thyroid hormones by the thyroid gland.



Hashimoto’s Thyroiditis

This is the most common causes of hypothyroidism in the developed world. It is caused by autoimmune inflammation of the thyroid gland. It is associated with antithyroid peroxidase (anti-TPO) antibodies and antithyroglobulin antibodies. Initially it causes a goitre after which there is atrophy of the thyroid gland.

Iodine Deficiency

This is the most common cause of hypothyroidism in the developing world. Iodine is added to foods such as table salt to prevent iodine deficiency.


Secondary to Treatment of Hyperthyroidism

All of the treatments for hyperthyroidism have the potential to cause hypothyroidism:

  • Carbimazole
  • Propylthiouracil
  • Radioactive iodine
  • Thyroid surgery



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.


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


Presentation and Features

  • Weight gain
  • Fatigue
  • Dry skin
  • Coarse hair and hair loss
  • Fluid retention (oedema, pleural effusions, ascites)
  • Heavy or irregular periods
  • Constipation



Primary Hypothyroidism is caused by thyroid gland insufficiency. Thyroid hormones (i.e. free T3 and T4) will be low. TSH will be high because there is no negative feedback to the brain, so the pituitary produces lots of TSH to try and get the thyroid working.

Secondary Hypothyroidism is caused by pituitary pathology that results in low production of TSH. Thyroid hormones will be low due to the low TSH.

Thyroid Status

Site of Pathology


T3 and T4

Primary Hypothyroidism

Thyroid Gland



Secondary Hypothyroidism

Pituitary Gland





Replacement of thyroid hormone with oral levothyroxine is the treatment of hypothyroidism. Levothyroxine is synthetic T4, and metabolises to T3 in the body. The dose is titrated until TSH levels are normal. When starting levothyroxine, initially measure TSH levels monthly until stable, then once stable it can be checked less frequently unless they become symptomatic.

If the TSH level is high, the dose is too low and needs to be increased. If the TSH is low, the dose is too high and needs to be reduced.


Last updated November 2018
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