Hypothyroidism: Causes, Thyroid Function Tests and Levothyroxine

Hypothyroidism means an underactive thyroid gland that produces too little thyroid hormone. It is common, especially in women, and its symptoms develop slowly, so it is easily missed. It is easy to diagnose with a blood test and easy to treat.

Causes

• Hashimoto's (autoimmune) thyroiditis — the most common cause where iodine intake is adequate; associated with anti-TPO antibodies.
• Iodine deficiency — the most common cause worldwide, still seen in parts of South Asia.
• After treatment of hyperthyroidism (radioactive iodine or surgery).
• Drugs: amiodarone, lithium, carbimazole.
• Transient thyroiditis, for example postpartum.
• Congenital hypothyroidism, detected by newborn screening.
• Secondary hypothyroidism from pituitary disease (rare). Here the TSH is low or normal.

Clinical features

Symptoms:

• tiredness
• weight gain
• feeling cold
• constipation
• dry skin
• hair thinning
• low mood
• poor memory
• menstrual changes (often heavy periods)
• carpal tunnel syndrome

Signs:

• bradycardia
• dry, coarse skin
• a puffy face
• loss of the outer third of the eyebrows
• a hoarse voice
• slow-relaxing reflexes
• goitre, in some causes

In children, untreated hypothyroidism impairs growth and development.

Investigations

• TSH and free T4:
  • Primary hypothyroidism: raised TSH, low free T4.
  • Subclinical hypothyroidism: raised TSH, normal free T4.
  • Secondary (pituitary) hypothyroidism: low or inappropriately normal TSH, low free T4.
• Thyroid peroxidase (TPO) antibodies: support autoimmune disease.
• Associated findings: raised cholesterol, macrocytosis, hyponatraemia and raised creatine kinase.

Treatment

• Levothyroxine, taken once daily on an empty stomach, ideally at the same time each day.
• Older people and those with ischaemic heart disease start on a low dose, increased gradually. Younger healthy adults can often start near the full replacement dose.
• Check TSH about 6–8 weeks after starting or changing the dose, aiming for a TSH within the reference range. Once stable, check it annually.
• Calcium and iron supplements reduce absorption, so take them several hours apart.
• Pregnancy: thyroid hormone requirements usually increase, so the dose is often raised early. Specialist guidance applies.

Subclinical hypothyroidism

Treatment depends on:

• the TSH level (often treated if TSH is 10 mU/L or more)
• symptoms
• age
• pregnancy or plans for pregnancy
• antibody status

Many patients only need monitoring.

Myxoedema coma

A rare but life-threatening emergency, usually in older patients with untreated hypothyroidism and a trigger such as infection, cold exposure or sedatives. Features:

• reduced consciousness
• hypothermia
• bradycardia
• hypotension
• hypoventilation
• hyponatraemia
• hypoglycaemia

It needs intensive care, IV thyroid hormone and IV hydrocortisone (until adrenal insufficiency is excluded), under specialist guidance.

Exam pearls

• TSH is the best single screening test for primary thyroid disease.
• Start low and go slow in elderly and cardiac patients.
• Amiodarone can cause both hypothyroidism and hyperthyroidism.

See also Hyperthyroidism and the endocrinology chapter of the Medicine Companion.

Test yourself

5 exam-style single-best-answer questions. Choose your answer, then tap “Show answer”.

1. TSH raised, free T4 low. Diagnosis?

   1. Hyperthyroidism
   2. Subclinical hyperthyroidism
   3. Sick euthyroid only
   4. Secondary hypothyroidism
   5. Primary hypothyroidism
   Show answer

   Answer: E. Primary hypothyroidism
   High TSH with low free T4 is primary hypothyroidism.

2. Most common cause of hypothyroidism worldwide:

   1. Lithium
   2. Pituitary tumour
   3. Hashimoto's thyroiditis
   4. Iodine deficiency
   5. Radioiodine
   Show answer

   Answer: D. Iodine deficiency
   Iodine deficiency is the commonest cause worldwide; Hashimoto's is commonest in iodine-replete areas.

3. 75-year-old with ischaemic heart disease and new hypothyroidism. Levothyroxine plan?

   1. Give T3 IV
   2. Start low and increase gradually
   3. Give carbimazole
   4. Avoid treatment
   5. Start full dose immediately
   Show answer

   Answer: B. Start low and increase gradually
   Start low and go slow in elderly and cardiac patients to avoid angina and arrhythmias.

4. When should TSH be rechecked after a dose change?

   1. Never
   2. 1 week
   3. 6 months
   4. 6–8 weeks
   5. 1 year
   Show answer

   Answer: D. 6–8 weeks
   TSH takes about 6–8 weeks to reflect a new steady state.

5. Which drug can cause both hypo- and hyperthyroidism?

   1. Amoxicillin
   2. Metformin
   3. Paracetamol
   4. Omeprazole
   5. Amiodarone
   Show answer

   Answer: E. Amiodarone
   Amiodarone contains iodine and can cause either hypothyroidism or thyrotoxicosis.

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Disclaimer

For education and exam preparation only. This is not medical advice and must not be used to diagnose or treat patients. Always follow current clinical guidelines and the advice of qualified senior colleagues.