Hypothyroidism affects around 1-2% of women in the UK and is around 5-10 times more common in females than males.

Primary hypothyroidism

Hashimoto's thyroiditis

• most common cause
• autoimmune disease, associated with IDDM, Addison's or pernicious anaemia
• may cause transient thyrotoxicosis in the acute phase
• 5-10 times more common in women

Subacute thyroiditis (de Quervain's)

Riedel thyroiditis

After thyroidectomy or radioiodine treatment

Drug therapy (e.g. lithium, amiodarone or anti-thyroid drugs such as carbimazole)

Dietary iodine deficiency

Secondary hypothyroidism (rare)

From pituitary failure

Other associated conditions

• Down's syndrome
• Turner's syndrome
• coeliac disease

General

• Weight gain
• Lethargy
• Cold intolerance

Skin

• Dry (anhydrosis), cold, yellowish skin
• Non-pitting oedema (e.g. hands, face)
• Dry, coarse scalp hair, loss of lateral aspect of eyebrows

Gastrointestinal

• Constipation

Gynaecological

• Menorrhagia

Neurological

• Decreased deep tendon reflexes
• Carpal tunnel syndrome

A hoarse voice is also occasionally noted.

Key points

• initial starting dose of levothyroxine should be lower in elderly patients and those with ischaemic heart disease. The BNF recommends that for patients with cardiac disease, severe hypothyroidism or patients over 50 years the initial starting dose should be 25mcg od with dose slowly titrated. Other patients should be started on a dose of 50-100mcg od
• following a change in thyroxine dose thyroid function tests should be checked after 8-12 weeks
• the therapeutic goal is 'normalisation' of the thyroid stimulating hormone (TSH) level. As the majority of unaffected people have a TSH value 0.5-2.5 mU/l it is now thought preferable to aim for a TSH in this range
• women with established hypothyroidism who become pregnant should have their dose increased 'by at least 25-50 micrograms levothyroxine'* due to the increased demands of pregnancy. The TSH should be monitored carefully, aiming for a low-normal value
• there is no evidence to support combination therapy with levothyroxine and liothyronine

Side-effects of thyroxine therapy

• hyperthyroidism: due to over treatment
• reduced bone mineral density
• worsening of angina
• atrial fibrillation

Interactions

• iron: absorption of levothyroxine reduced, give at least 2 hours apart

*source: NICE Clinical Knowledge Summaries