Aetiology

Autoimmune, post-radioiodine, post-surgery.

Diagnostic tests

Low free T₄ and elevated TSH (usually greater than 20mU/L).

Therapy

• Levothyroxine is the standard treatment. Liothyronine use is restricted to initiation by a consultant (See 6.2.1 Thyroid hormones).
• Levothyroxine should be used with caution in cardiac disease, severe hypothyroidism, and patients over 50 years. A lower starting dose is required for these patients (See 6.2.1 Thyroid hormones). See below for advice on managing patients with ischaemic heart disease.

Monitoring the effect of treatment

• Repeat thyroid function test every 4-6 weeks after the initiation of treatment. Once stable, yearly measurements are advisable to check compliance and if dose is still appropriate.
• The aim of therapy is to have the TSH in the normal range. Most patients feel symptomatically better with a low normal TSH (less than 2mU/L) and high normal free T₄.
• Symptomatic improvement begins in 3-4 weeks, but may not be complete for several months, especially when myopathic symptoms are present. Lifelong levothyroxine is required

Hypothyroidism should be managed very cautiously in patients with known ischaemic heart disease. Levothyroxine is likely to precipitate or exacerbate angina.

• If levothyroxine precipitates angina, further investigation and treatment of ischaemic heart disease is essential and urgent referral should be made to the thyroid or chest pain clinic.
• Consider beta-blocker, but most patients with angina when hypothyroid will not get symptomatic control on medication.

Resist the temptation to increase the levothyroxine above 150 or 200 micrograms to treat fatigue - especially if the TSH is suppressed. Iatrogenic 'apathetic' thyrotoxicosis will worsen symptoms.

Aetiology

Autoimmune, post-radioiodine, post-surgery.

Diagnostic test

Normal free T₄, elevated TSH (greater than 4.5 mU/L).

Symptoms

25-50% of these patients feel better when taking levothyroxine.

Prognosis

About 5% per annum progress to overt clinical hypothyroidism.

Predictors of progression = high titre thyroid peroxidise antibodies (greater than 1:1600) and age (over 65 years).

Therapy

TSH greater than 10 mU/L - prevent progression by initiating levothyroxine therapy.

TSH less than 10 mU/L with thyroid peroxidase (TPO) antibodies – repeat thyroid function annually.

TSH less than 10 mU/L without TPO antibodies – repeat thyroid function every 3 years.

If TSH greater than 4.5 mU/L consider treatment if symptoms or high titres of TPO antibodies.

Aetiology

Hypothalamic or pituitary disease.

Diagnosis

Hypothyroidism secondary to pituitary disease will often not be detected by the routine testing as TSH (the screening test) is often normal. This means if a diagnosis of secondary hypothyroidism is considered, this should be marked on the request form and a free T₄ also requested. Hypopituitarism is the commonest cause of secondary hypothyroidism, so if this diagnosis is considered, the patient should be referred to the thyroid/endocrine clinic for full assessment of pituitary function before levothyroxine replacement is instituted. Treating secondary hypothyroidism in hypopituitary patients can precipitate an "adrenal" crisis.

Hypothyroidism in pregnancy is associated with maternal and foetal complications and should be treated.

In patients with pre-existing hypothyroidism, most patients will require an increased dose of levothyroxine to keep the TSH ideally less than 2.5mU/L. This increase often occurs early in the first trimester.

Patients with known hypothyroidism should increase the dose of levothyroxine by 50micrograms daily as soon as pregnancy is confirmed. They should have their thyroid function tested early in the first trimester and then 6-8 weekly throughout the pregnancy.