Hypothyroidism

Definition

Hypothyroidism is a condition characterised by an underactive thyroid gland, which fails to produce sufficient thyroid hormones (thyroxine (T4) and triiodothyronine (T3)) to meet the body’s needs. This results in a slowdown of metabolic processes and various physiological functions.

Aetiology

Hypothyroidism can be caused by various factors:

Primary Hypothyroidism: Due to intrinsic thyroid gland dysfunction. Common causes include Hashimoto’s thyroiditis (autoimmune), iodine deficiency, and thyroid surgery.
Secondary Hypothyroidism: Due to pituitary gland dysfunction, leading to insufficient thyroid-stimulating hormone (TSH) production.
Tertiary Hypothyroidism: Due to hypothalamic dysfunction, leading to insufficient thyrotropin-releasing hormone (TRH) production.

Pathophysiology

Thyroid hormones (T4 and T3) regulate various metabolic processes, including basal metabolic rate, protein synthesis, and sympathetic nervous system activity. In hypothyroidism, decreased levels of these hormones result in reduced metabolism, leading to symptoms such as fatigue, weight gain, and cold intolerance. The negative feedback mechanism involving the hypothalamus and pituitary gland is disrupted, often resulting in elevated TSH levels in primary hypothyroidism.

Risk Factors

Female gender
Age over 60 years
Family history of thyroid disease
Autoimmune diseases (e.g., type 1 diabetes, rheumatoid arthritis)
Previous thyroid surgery or radiation therapy
Iodine deficiency or excess
Certain medications (e.g., lithium, amiodarone)

Signs and Symptoms

Common signs and symptoms of hypothyroidism include:

Fatigue and weakness
Weight gain
Cold intolerance
Constipation
Dry skin and hair
Hair loss
Bradycardia (slow heart rate)
Depression
Memory problems and cognitive impairment
Muscle aches and stiffness
Menstrual irregularities

Investigations

Serum TSH: Elevated in primary hypothyroidism and low or normal in secondary/tertiary hypothyroidism
Free T4: Low in hypothyroidism
Thyroid autoantibodies: To diagnose autoimmune thyroiditis (e.g., anti-thyroid peroxidase (TPO) antibodies)
Thyroid ultrasound: To assess thyroid gland structure and detect nodules
Radioactive iodine uptake test: To evaluate thyroid function and iodine uptake

Management

Primary Care Management

Levothyroxine replacement therapy: The mainstay of treatment to normalise thyroid hormone levels. The dose is adjusted based on regular monitoring of TSH and free T4 levels.
Monitoring: Regular follow-up to assess symptoms and biochemical markers (TSH and free T4 levels).
Education: Informing patients about the condition, the importance of medication adherence, and recognising symptoms of under- or over-treatment.

Specialist Management

Referral to endocrinologist: For complex cases, including those with refractory symptoms or difficulty achieving target hormone levels.
Management of associated conditions: Addressing comorbidities such as cardiovascular disease, hyperlipidaemia, and autoimmune disorders.
Additional imaging or tests: If there are concerns about thyroid nodules or malignancy, further imaging or fine-needle aspiration biopsy may be required.

Example Management for Hypothyroidism

A patient diagnosed with primary hypothyroidism presenting with fatigue, weight gain, and elevated TSH should be started on levothyroxine replacement therapy. The initial dose is typically based on the patient’s weight, age, and cardiac status. Regular monitoring of TSH and free T4 levels is essential to adjust the dose and achieve euthyroid status. The patient should be educated on the importance of taking the medication consistently and informed about potential symptoms of over- or under-treatment. Referral to an endocrinologist may be considered for complex cases or if the patient has associated autoimmune conditions.

References

NICE. (2024). Thyroid Disease: Assessment and Management. Retrieved from NICE
NHS. (2023). Hypothyroidism (Underactive Thyroid). Retrieved from NHS
British Thyroid Association (BTA). (2022). Guidelines for the Management of Hypothyroidism. Retrieved from BTA
Endocrine Society. (2021). Clinical Practice Guidelines for Hypothyroidism. Retrieved from Endocrine Society