Hyperthyroidism has a significant short- term morbidity and long- term morbidity and mortality. In epidemiological studies, the clinical diagnosis of thyrotoxicosis should be supported by measurements of serum T4 or T3 and thyrotropin (TSH) concentrations. Biochemical tests of thyroid function may reveal the diagnosis before it is clinically apparent. A rise in serum T3 and fall in serum TSH are the earliest measures of thyroid overactivity, followed by a rise in serum T4. The most common cause of hyperthyroidism in iodine- replete com munities is Graves’ disease, followed by toxic multinodular goitre, whereas rarer causes include an autonomously functioning thyroid adenoma, autoimmune or viral thyroiditis, excessive levothyroxine (L- T4) replacement or drugs including amiodarone and lithium carbonate. In epidemiological studies, the aetiology is rarely ascertained.

Prevalence of Hyperthyroidism

The prevalence of hyperthyroidism in women is between 0.5 and 2% and is ten times more common in women than in men in iodine- replete communities. In the Whickham survey in Northeast England, the prevalence of undiagnosed hyperthyroidism was 4.7/ 1000 women. Hyperthyroidism had been previously diagnosed and treated in 20/ 1000 women, rising to 27/ 1000 women when possible but unproven cases were included, as compared with 1.6 to 2.3/ 1000 men, in whom no new cases were found at the survey. The mean age at diagnosis was 48 years. In the US National Health and Nutrition Examination Survey (NHANES III), of those subjects who were neither taking thy roid medication nor reported a history of thyroid disease, 2/ 1000 had ‘clinically significant’ hyperthyroidism, defined as a serum TSH concentration less than 0.1 mU/ L and serum total T4 concentration greater than 170 nmol/ L. A cross- sectional survey of 25 682 subjects aged over 18 years attending a Health Fair in Colorado, US found that overt hyperthyroidism, defined as serum TSH concentration less than 0.01 mU/ L, was present in only 1/ 1000 of those not taking thyroid medication. A higher prevalence is seen in iodine- deficient areas. Prevalence data in elderly persons show a wide range between 0.4 and 2.0%. The reported prevalence rates for previously undiagnosed hyperthyroidism in hospitalized patients is between 0.3 and 1% and consistent with community surveys.

Subclinical Hyperthyroidism

The introduction of assays for serum TSH that were sensitive enough to distinguish between normal and low concentrations allowed subjects with subclinical hyperthyroidism to be identified. Subclinical hyperthyroidism is defined as a low serum TSH concentration and normal serum T4 and T3 concentrations, in the absence of hypothalamic or pituitary disease, non- thyroidal illness, or in gestion of drugs that inhibit TSH secretion such as glucocorticoids or dopamine. Epidemiological studies differ in the definition of a low serum TSH concentration and whether the subjects included were receiving L- T4 therapy.

The reported overall prevalence ranges from 0.5 to 6.3%, with men and women over 65 years having the highest prevalence and approximately half taking L- T4. The Colorado study of 25 682 healthy volunteers (of whom 88% were white) found 2% had a subnormal serum TSH, with more than half on L- T4. In the NHANES III study the prevalence was highest in those subjects aged 20 to 39 years and those aged greater than 79 years. The percentage with serum TSH concentrations less than 0.4 mU/ L was significantly higher in women than men, and black subjects had a higher prevalence (0.4%) than whites (0.1%) or Mexican Americans (0.3%). The prevalence is higher in iodine- deficient populations due to functional autonomy from nodular goitres (Table 1).

Table1. The effect of environmental iodine intake on the prevalence of subclinical thyroid disease

Among subjects with subclinical hyperthyroidism, those with low but detectable serum TSH values may recover spontaneously when retested [13]. Non- thyroidal illness is an important cause of false- positive serum TSH test results. There are limited data on the risk of progression of subclinical hyperthyroidism to overt hyperthyroidism. T hose with an undetectable serum TSH and confirmed aetiology as determined by thyroid scintigraphy due to Graves’ disease or nodular disease have an annual incidence of approximately 5– 8%. A population study in Tayside, United Kingdom (UK) followed 2024 subjects with at least two serum TSH measurements below the reference range for at least four months for up to seven years. Few subjects developed hyperthyroidism (0.5– 0.7%) and the percentage of those re verting to normal increased with time and was more common if the baseline serum TSH was between 0.1 and 0.4 mU/ L.

Incidence of Hyperthyroidism

 The incidence data available for overt hyperthyroidism in men and women from large population studies are comparable, at 0.4/ 1000 women and 0.1/ 1000 men, but the age- specific incidence varies considerably. The peak age- specific incidence of Graves’ dis ease was between 20 and 49 years in two studies but increased with age in Iceland and peaked at 60 to 69 years in Malmö, Sweden. The peak age- specific incidence of hyperthyroidism caused by toxic nodular goitre and autonomously functioning thyroid adenomas in the Malmö study was over 80 years. The only available data in a black population, from Johannesburg, South Africa, suggest a tenfold lower annual incidence of hyperthyroidism (0.09/ 1000 women and 0.007/ 1000 men) than in whites.

In the 20- year follow- up of the Whickham cohort, the mean annual incidence of hyperthyroidism in women was 0.8/ 1000 survivors (95% confidence interval (CI), 0.5– 1.4). The incidence rate was similar in the deceased women. No new cases were detected in men. An estimate of the probability of the development of hyperthyroidism in women at a particular time averaged 1.4/ 1000 between the ages of 35 and 60 years (Figure 1). Serum antithyroid antibody status or goitre was not associated with the development of hyperthyroidism at follow- up. Other cohort studies provide comparable incidence data, which suggests that many cases of hyperthyroidism remain undiagnosed in the community unless routine testing is undertaken. In Tayside, UK, 620 incident cases of hyperthyroidism were identified from medical records with an incidence rate of 0.77/ 1000 per year (95% CI, 0.70– 0.84) in women and 0.14/ 1000 per year (95% CI, 0.12– 0.18) in men. The incidence increased with age, and women were affected two to eight times more than men across the age range. The incidence increased in women but not men between 1997 and 2001.

Fig1. Age- specific hazard rates for the development of overt hyperthyroidism and hypothyroidism in women at 20- year follow- up of the Whickham survey. Reproduced with permission from Vanderpump MPJ, Tunbridge WMG, French JM, Appleton D, Bates D, Clark F, et al. The incidence of thyroid disorders in the community: a twenty- year follow- up of the Whickham survey. Clin Endocrinol, 1995; 43: 60. Copyright © 2008, John Wiley and Sons, Blackwell Science.

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مواضيع ذات صلة

Thyroid Disease in Pregnancy

Hypothyroidism

Clinical Manifestations of Iron Deficiency

Thyroid Imaging: nuclear Medicine Techniques - Thyroiditis

Thyroid Imaging: nuclear Medicine Techniques - Thyrotoxicosis

Pathobiology of Iron Deficiency

Outcome and Prevention of Infectious Diseases

Serum and Tissue Thyroid Hormone concentrations in NTI

Microorganism Entry, Invasion, and Dissemination: The Host’s Perspective

Microorganism Colonization of Host Surfaces

The Encounter Between Host and Microorganism

Systemic Approach to Anemia