12th April 2024

Hypothyroidism

Hypothyroidism is the result of a low level of thyroid hormones in circulation. The causes of hypothyroidism can be acquired or inherited. Worldwide, the most common cause of hypothyroidism is a result of malnutrition, leading to iodine deficiency. In the UK, autoimmune diseases such as Hashimoto’s disease are the leading cause of hypothyroidism. Other causes include postpartum thyroiditis, De Quervain disease, medications (amiodarone, phenytoin, lithium), iatrogenic (surgical/radiotherapy) and tumours. The pathophysiology of hypothyroidism can be divided into two main groups: primary and secondary. Primary causes are when the ability to produce thyroid hormones from the thyroid gland itself is affected, while secondary causes are when the hypothalamus-pituitary gland is affected. Typical signs and symptoms are a result of low metabolism and low cell turnover. Low metabolism can lead to fatigue, weight gain, sensitivity to cold, reduced reflexes and bradycardia. Low cell turnover results in brittle hair and thin skin. Autoimmune conditions can lead to thyroid enlargement (goitres).

Investigations

Diagnosis of hypothyroidism is when T4 levels are <12pmol/L. Differentiating the type of hypothyroidism relies on TSH levels. Primary causes are due to low production from the thyroid glands, resulting in high TSH levels to try and stimulate more production. Secondary causes are a result of a reduction or blockage of stimulation of the thyroid gland and so will typically have low TSH levels. Subclinical hypothyroidism has the potential to lead to hypothyroidism and will typically have high TSH levels with compensated T4 levels. Thyroid autoantibodies, including anti-thyroid peroxidase and anti-thyroglobulin antibodies, suggest Hashimoto thyroiditis. Adrenal insufficiency should be investigated concurrently with hypothyroidism due to the potential of exacerbation with levothyroxine treatment if left untreated.

Management

Management of hypothyroidism mainly involves the use of levothyroxine as hormone replacement. Initial doses should be around 1.6mcg/kg; however, if the patient is elderly or has a history of cardiac diseases, lower doses should be used due to the risk of developing angina or AF.