Underactive thyroid (hypothyroidism)

Definition
Hypothyroidism is a condition in which the thyroid gland produces insufficient amounts of thyroid hormones (thyroxine/T₄ and triiodothyronine/T₃), leading to a general slowing of metabolic processes in the body.

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Types

1. Primary hypothyroidism – failure of the thyroid gland itself (most common)

2. Secondary hypothyroidism – due to pituitary dysfunction (low TSH)

3. Tertiary hypothyroidism – due to hypothalamic disease (low TRH)

4. Subclinical hypothyroidism – raised TSH with normal free T₄; often asymptomatic but may progress to overt disease

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Causes

Primary causes

• Autoimmune thyroiditis (Hashimoto’s thyroiditis) – most common in developed countries

• Iatrogenic – post-thyroidectomy, post-radioiodine therapy, neck irradiation

• Drug-induced – amiodarone, lithium, interferon-α, tyrosine kinase inhibitors

• Iodine deficiency (common in developing countries)

• Congenital hypothyroidism (thyroid dysgenesis, dyshormonogenesis)

• Subacute or chronic thyroiditis

Secondary/Tertiary causes

• Pituitary tumours or surgery

• Hypothalamic tumours, trauma, or radiation

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Risk Factors

• Female sex

• Age >60 years

• Family history of thyroid disease or autoimmune disease

• Previous neck surgery or irradiation

• Use of drugs affecting thyroid function

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Clinical Features

General/metabolic

• Fatigue, lethargy

• Weight gain (despite poor appetite)

• Cold intolerance

• Slow movements and speech

Skin and hair

• Dry, coarse skin

• Hair thinning, loss of outer third of eyebrows

• Pallor, puffiness

Cardiovascular

• Bradycardia

• Low cardiac output

• Pericardial effusion in severe cases

Gastrointestinal

• Constipation

Neuromuscular

• Muscle weakness and cramps

• Slow reflex relaxation

• Carpal tunnel syndrome

Reproductive

• Menorrhagia or oligomenorrhoea

• Infertility

Neuropsychiatric

• Depression, poor memory, slow thinking

• Severe form: myxoedema coma (rare, life-threatening)

Goitre – in autoimmune or iodine-deficiency cases

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Diagnosis

• TSH – high in primary hypothyroidism, low/normal in secondary

• Free T₄ – low in overt hypothyroidism

• Thyroid antibodies – anti-thyroid peroxidase (anti-TPO) positive in Hashimoto’s

• Lipid profile – hypercholesterolaemia common

• Other tests: ECG (bradycardia), hyponatraemia on bloods in severe disease

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Treatment

1. Levothyroxine (synthetic T₄) – mainstay of therapy

• Typical adult starting dose: 1.6 µg/kg/day orally (lower starting doses in elderly or those with cardiac disease: 25–50 µg/day)

• Taken once daily, preferably on an empty stomach

• Adjust dose every 6–8 weeks according to TSH (primary) or free T₄ (secondary)

• Lifelong therapy in most cases

2. Special considerations

• Pregnancy: Increase dose by 25–50 µg/day as soon as pregnancy confirmed; monitor TSH every 4–6 weeks

• Myxoedema coma: Emergency – IV levothyroxine and hydrocortisone, supportive care in ICU

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Prognosis

• Excellent with appropriate replacement therapy

• Untreated disease can lead to cardiovascular disease, infertility, and severe metabolic decompensation (myxoedema coma)