Hypothyroidism

Hypothyroidism refers to insufficient thyroid hormones, triiodothyronine (T3) and thyroxine (T4).

Primary hypothyroidism is where the thyroid behaves abnormally and produces inadequate thyroid hormones. Negative feedback is absent, resulting in increased production of TSH. TSH is raised, and T3 and T4 are low.

Secondary hypothyroidism, also called central hypothyroidism, is where the pituitary behaves abnormally and produces inadequate TSH, resulting in under-stimulation of the thyroid gland and insufficient thyroid hormones. TSH, T3 and T4 will all be low.

Causes of Primary Hypothyroidism

Hashimoto’s thyroiditis is the most common cause of hypothyroidism in the developed world. It is an autoimmune condition causing inflammation of the thyroid gland. It is associated with anti-thyroid peroxidase (anti-TPO) antibodies and anti-thyroglobulin (anti-Tg) antibodies.

Iodine deficiency is the most common cause of hypothyroidism in the developing world. In the UK, iodine is particularly found in dairy products and may be added to non-dairy milk alternatives (e.g., soya milk). 

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 can also cause thyrotoxicosis.

Causes of Secondary Hypothyroidism

Secondary hypothyroidism is often associated with a lack of other pituitary hormones, such as ACTH, referred to as hypopituitarism. This is rarer than primary hypothyroidism, and may be caused by:

• Tumours (e.g., pituitary adenomas)
• Surgery to the pituitary
• Radiotherapy
• Sheehan’s syndrome (where major post-partum haemorrhage causes avascular necrosis of the pituitary gland)
• Trauma

Presentation

The features that are universal to all causes of hypothyroidism include:

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

Iodine deficiency causes a goitre. 

Hashimoto’s thyroiditis can initially cause a goitre, after which there is atrophy (wasting) of the thyroid gland.

Management

Oral levothyroxine is the mainstay of treatment of hypothyroidism. Levothyroxine is a synthetic version of T4 and metabolises to T3 in the body. 

The dose is titrated based on the TSH level, initially every 4 weeks.

Liothyronine sodium is a synthetic version of T3 and is very rarely used under specialist care where levothyroxine is not tolerated.

Last updated March 2023