Aetiology

Autoimmune, post-radioiodine, post-surgery.

Diagnostic tests

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

Therapy

• In young patients (below the age of 60 years) without ischaemic heart disease, start levothyroxine at a daily dose of 1.6 microgramms/kg body weight (equates to 100 micrograms daily for the average sized women (60kg) and 125 micrograms daily for the average sized man (75kg). 85% of patients are euthyroid on 100-150 micrograms.
• Levothyroxine should be replaced cautiously in older patients (over 60 years) and those with ischaemic heart disease. The starting dose of levothyroxine in these instances should be 25 micrograms daily.
• The long half-life means that if a dose is missed it may be taken the next day.
• There is not sufficient evidence at present to support the routine use of T₃ which is harder to monitor.

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 IHD is mandatory 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 4.5mu/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 10mu/L with TPO antibodies – repeat thyroid function annually.

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

If TSH greater than 4.5mu/L consider treatment if symptoms or high titres of thyroid peroxidase 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 fetal 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 50mcg 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.