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Hypothyroidism (underactive thyroid) is an endocrine disorder where the thyroid gland does not produce enough thyroid hormones. Symptoms include extreme fatigue, cold intolerance, muscle aches, constipation, weight gain, and depression.

Goitre (swelling of the neck) may occur due to an enlarged thyroid gland. Untreated hypothyroidism during pregnancy can lead to developmental issues in the baby.

Molecular structure of thyroxine, which is deficient in hypothyroidism
Molecular structure of thyroxine, which is deficient in hypothyroidism


The most common global cause of hypothyroidism is iodine deficiency. In areas with sufficient iodine, Hashimoto's thyroiditis is the leading cause.

Other causes include previous thyroid surgery, radioactive iodine treatment, certain medications, and congenital defects. The condition is diagnosed through blood tests measuring thyroid-stimulating hormone (TSH) and thyroxine levels.

Signs and Symptoms

Hypothyroidism can present with a variety of symptoms, often mild or non-specific:

  • Common symptoms: Extreme fatigue, cold intolerance, muscle aches, constipation, weight gain, and depression.
  • Specific signs: Dry skin, slow heart rate, and swelling of limbs.
  • Severe complications: Myxedema coma, a life-threatening condition characterised by low body temperature, confusion, and slow heart rate.
Symptoms and signs of hypothyroidism
Symptoms and signs of hypothyroidism


Diagnosis is primarily through blood tests:

  • TSH Test: Elevated TSH indicates hypothyroidism.
  • T4 Test: Low thyroxine levels confirm the diagnosis.

Other diagnostic measures include checking for thyroid peroxidase antibodies to identify autoimmune thyroiditis. Imaging is usually unnecessary unless a thyroid mass is detected.


The main treatment for hypothyroidism is hormone replacement therapy with levothyroxine, adjusted based on symptoms and blood test results. In central hypothyroidism, treatment decisions are based on free T4 levels rather than TSH.

Special Considerations

  • Pregnancy: Pregnant women with hypothyroidism require careful monitoring and dose adjustments to maintain appropriate thyroid hormone levels.
  • Subclinical Hypothyroidism: This is characterised by elevated TSH with normal thyroxine levels. Treatment is debated, often based on symptoms, TSH levels, and presence of antibodies.


Iodine supplementation through iodised salt has significantly reduced the incidence of hypothyroidism in many parts of the world. Pregnant and breastfeeding women require higher iodine intake, and supplements are often recommended.

A 3-month-old infant with untreated congenital hypothyroidism
A 3-month-old infant with untreated congenital hypothyroidism showing myxedematous facies, a big tongue, and skin mottling


Hypothyroidism affects 0.3–0.4% of the population in iodine-sufficient countries. Subclinical hypothyroidism is more common, affecting up to 8.5%.

Women and older adults are more frequently affected. In regions with iodine deficiency, hypothyroidism is more prevalent due to dietary insufficiency.

History and Other Animals

The link between iodine and thyroid function was established in the 19th century. Hypothyroidism treatment evolved from animal thyroid extract to synthetic hormones.

Characteristic changes in the facial skin of a Labrador Retriever with hypothyroidism
Characteristic changes in the facial skin of a Labrador Retriever with hypothyroidism

In veterinary practice, hypothyroidism is most common in dogs, particularly due to lymphocytic thyroiditis or idiopathic atrophy. Treatment involves thyroid hormone replacement, similar to human therapy.

Self-assessment MCQs (single best answer)

What is the most common cause of hypothyroidism in areas with sufficient dietary iodine?

Which of the following symptoms is most commonly associated with hypothyroidism?

What is the preferred initial test for diagnosing hypothyroidism?

Which medication is most commonly used to treat hypothyroidism?

What complication can untreated hypothyroidism during pregnancy lead to in the baby?

Which physical sign is characteristic of severe hypothyroidism (myxedema)?

What is the main component of the hormone replacement therapy for hypothyroidism?

Which of the following is NOT a typical symptom of hypothyroidism?

In the context of hypothyroidism, what does an elevated TSH level with a low T4 level indicate?

Which population is most commonly affected by hypothyroidism?


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Brilliant videos, thank you.

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