Hypothyroidism

Definition/diagnostic criteria Hypothyroidism is characterised by an inadequate production of thyroid hormones, specifically thyroxine (T4) and triiodothyronine (T3).

In overt hypothyroidism, thyroid-stimulating hormone (TSH) levels are above the normal reference range (usually 10mU/L) and free thyroxine (T4) is below the normal reference range.

In subclinical hypothyroidism TSH levels are above the normal reference range, but T3 and T4 levels are within the normal reference range.

In secondary hypothyroidism, TSH levels are low or normal and FT4 is below the normal reference range.

Epidemiology Hypothyroidism is a prevalent condition in the UK. The prevalence of overt hypothyroidism is approximately 3% in women and 0.1% in men. Subclinical hypothyroidism affects around 8% of women and 2% of men.

Primary hypothyroidism may be caused by autoimmune thyroiditis (95% of cases), iodine deficiency, post-ablative therapy or surgery, drugs (such as amiodarone and lithium), transient thyroiditis, and thyroid infiltrative disorders.

Secondary hypothyroidism is caused by a pituitary or hypothalamic disorder.

Diagnosis

Clinical features

These may be mild and non-specific but can include:

• Fatigue and lethargy
• Weight gain and fluid retention
• Constipation
• Cold intolerance
• Dry skin and hair loss (e.g. loss of lateral eyebrows)
• Hoarseness and voice changes
• Goitre
• Menstrual irregularities, infertility or subfertility
• Muscle aches and weakness
• Depression and cognitive impairment
• Thyroid pain e.g. in subacute (De Quervain’s) thyroiditis
• Carpal tunnel syndrome

Investigations

• Thyroid function tests (TFTs): Elevated TSH levels with low FT4 levels indicate primary hypothyroidism. In subclinical hypothyroidism TSH is raised with a normal FT4. In secondary hypothyroidism both TSH and FT4 are typically low.
• Antithyroid antibodies: Testing for thyroid peroxidase antibodies (TPOAb) if autoimmune thyroid disease is suspected.
• Thyroid ultrasound: If there are hard or irregular nodules or goitre which are suspicious for malignancy.
• If primary hypothyroidism is confirmed, consider blood tests for associated conditions (FBC and B12 for pernicious anaemia, serology for coeliac’s disease, HbA1c for diabetes mellitus, blood lipids).

Treatment If starting thyroxine, advise the person to take this medication on an empty stomach in the morning before other food or medication.

Overt hypothyroidism

• Start treatment with levothyroxine monotherapy (LT4) . For adults aged 18–49 years — initially 50–100 micrograms once daily; adjusted in steps of 25–50 micrograms every 3–4 weeks, adjusted according to response; maintenance 100–200 micrograms once daily. For adults aged 50 years and over, with cardiovascular disease, or severe hypothyroidism — initially 25 micrograms once daily; adjusted in steps of 25 micrograms every 4 weeks, adjusted according to response; maintenance 50–200 micrograms once daily.
• Review symptoms and TFTs every 3 months and make dose adjustments until TSH is stable
• Check TSH annually once stable.
• Check for underlying causes if TFTs remain abnormal or there are persistent symptoms despite adequate or escalating LT4 doses, and considering referral to endocrinology if no cause is found.

Subclinical hypothyroidism

• Start treatment with LT4 monotherapy depending on the TSH level, the person’s age, and presence of symptoms.
• Review symptoms and TFTs every 3 months and making LT4 dose adjustments if needed.
• Measure TSH and FT4 appropriately if the person is untreated or has stopped LT4.

Overt or subclinical hypothyroidism in a woman who is pregnant

• urgent TFTs are needed and the LT4 dose should be adjusted on specialist advice)

Overt or subclinical hypothyroidism in a woman who is planning a pregnancy or post-partum

• Referral to an endocrinology specialist should be arranged

Secondary hypothyroidism

• Urgent referral to an endocrinologist

Suspicion of myxoedema coma

• Emergency admission

Prognosis Complications of untreated or undertreated hypothyroidism include dyslipidaemia, metabolic syndrome, coronary heart disease and stroke, heart failure, neurological and cognitive impairments, and adverse maternal and fetal outcomes in pregnancy.

It is vital to educate patients about the need for ongoing monitoring and adherence to treatment.