hypothyroidism

Last edited 04/2021 and last reviewed 10/2023

Hypothyroidism describes the clinical syndrome which results from a deficiency of circulating thyroid hormones; myxoedema is a severe form of this syndrome and usually applied to cases in which deposition of mucinous substances results in thickening of the skin and subcutaneous tissues.

Hypothyroidism can be classified according to:

• the time of onset
• congenital
• acquired (1)
• the level of endocrine dysfunction as
• primary hypothyroidism - abnormality in thyroid gland itself
• secondary hypothyroidism - secondary to disorders of the pituitary or hypothalamus (1,2)
• the severity as
• overt hypothyroidism - TSH concentration is above the reference range (greater than10mU/L) & serum free T4 levels below the reference range
• subclinical hypothyroidism - serum TSH concentration is increased, serum free T4 concentrations are within the reference range (2)

The prevalence of spontaneous hypothyroidism in UK is approximately between 1%-2% with women affected 10 times more than men.

• a survey in the north east England have shown that the
  • prevalence of newly diagnosed overt hypothyroidism was 3 per 1000 women
  • 8% of women (with 10% of women over the age of 55) and 3% of men had subclinical hypothyroidism
• in community studies carried out on elderly people, 10% over 60 years had serum TSH values above the reference range which confirms the high prevalence of subclinical hypothyroidism in this age group (2)
• however, any age group may be affected.

Onset is extremely insidious and symptoms may be unnoticed for several years. Lethargy, fatigue and physical and mental slowness are common presenting features in adults; in children, severe learning disability may result (3).

In countries that are iodine sufficient, the prevalenceof overt hypothyroidism ranges from 1% to 2%, rising to 7% in older adults (4).

In people with subclinical hypothyroidism who have positive TPO antibody titres (4):

• 4.3% go on to develop overt hypothyroidism each year, compared with 2.6% of people with subclinical hypothyroidism but negative TPO antibody titres
• patients with subclinical hypothyroidism and positive TPO antibodies should be monitored more closely, with TSH checked every 6-12 months
• in some patients with subclinical hypothyroidism, TSH will normalise spontaneously

Notes (3):

• it may take several months before symptoms of hypothyroidism are resolved after biochemical correction of hypothyroidism
• if thyroid stimulating hormone level is persistently raised after an adequate dose of levothyroxine, suspect poor compliance (concordance), the presence of drug interference, or malabsorption (for example, undiagnosed coeliac disease)
• if a new drug is started, think whether the drug would interfere with thyroxine absorption or thyroid hormone action; ferrous and calcium salts are common culprits
• monitoring replacement with serum thyroid stimulating hormone alone is adequate in most patients with hypothyroidism; the important exception being those with pituitary or hypothalamic disease

Reference:

• (1) Roberts CG, Ladenson PW. Hypothyroidism. Lancet. 2004;363(9411):793-803
• (2) Association for Clinical Biochemistry (ACB), British Thyroid Association (BTA), British Thyroid Foundation (BTF) 2006. UK guidelines for the use of thyroid function tests
• (3) Vaidya B, Pearce SH. Management of hypothyroidism in adults. BMJ. 2008;337:a801
• (4) Siskind SM et al. Investigating hypothyroidism. BMJ 2021;373:n993http://dx.doi.org/10.1136/bmj.n993