Hypothyroidism

Definition | Aetiology | Pathophysiology | Risk Factors | Signs and Symptoms | Investigations | Management

Definition

Hypothyroidism is a condition characterised by insufficient production of thyroid hormones (T3 and T4), leading to a decreased metabolic rate and systemic effects.

Aetiology

  • Primary hypothyroidism: caused by thyroid gland dysfunction (e.g., Hashimoto’s thyroiditis, iodine deficiency, post-thyroidectomy, radioactive iodine therapy).
  • Secondary hypothyroidism: due to pituitary dysfunction leading to reduced TSH production.
  • Tertiary hypothyroidism: caused by hypothalamic dysfunction leading to reduced TRH production.
  • Drug induced: caused by lithium, amiodarone, interferon therapy.

Pathophysiology

  • Thyroid hormone deficiency leads to reduced metabolic activity in multiple organ systems.
  • In primary hypothyroidism, low T4 leads to elevated TSH as a compensatory response.
  • Secondary and tertiary hypothyroidism result in inadequate stimulation of the thyroid gland due to deficient TSH or TRH.

Risk Factors

  • Female sex.
  • Family history of thyroid disorders.
  • Autoimmune diseases (e.g., type 1 diabetes, coeliac disease, Addison’s disease).
  • Previous thyroid surgery or radioactive iodine therapy.
  • Use of medications that affect thyroid function.

Signs and Symptoms

  • General: fatigue, weight gain, cold intolerance.
  • Neurological: depression, cognitive impairment, slow reflexes.
  • Dermatological: dry skin, brittle hair, hair thinning.
  • Cardiovascular: bradycardia, hypotension.
  • Gastrointestinal: constipation.
  • Reproductive: menstrual irregularities, infertility.
  • Thyroid examination: goitre in Hashimoto’s thyroiditis, atrophic thyroid in late disease.

Investigations

  • Thyroid function tests: high TSH, low free T4 in primary hypothyroidism; low TSH and low T4 in secondary hypothyroidism.
  • Thyroid autoantibodies: anti thyroid peroxidase (TPO) and anti thyroglobulin antibodies in Hashimoto’s thyroiditis.
  • Serum lipid profile: commonly shows hyperlipidaemia.
  • ECG: may show bradycardia and low voltage QRS complexes.

Management

1. Hormone Replacement Therapy:

  • Levothyroxine: first line treatment, titrated based on TSH levels.
  • Monitor TSH every 6–8 weeks initially, then annually once stable.

2. Symptomatic Management:

  • Supportive treatment for fatigue, weight gain, and constipation.
  • Treat associated hyperlipidaemia if present.

3. Referral:

  • Endocrinology: for unstable cases, secondary hypothyroidism, or suspected malignancy.
  • Cardiology: if there are significant cardiovascular complications.
EndocrinologymypanotesHypothyroidism