Introduction: Subclinical hyperthyroidism (SH, grade 1, thyrotropin (TSH) ≥0.1 mU/L and grade 2, TSH <0.1 mU/L) is a common disorder with increased prevalence in older subjects. There is evidence for increased morbidities in SH, such as atrial fibrillation and osteoporosis. We aim to study the natural history and comorbidities of SH from patients referred to a tertiary endocrine clinic in Singapore as they are currently unknown.Materials and Methods: Retrospective evaluation of SH subjects for natural progression and comorbidities. Results: One hundred and thirteen SH subjects (male/female: 24/89, mean age: 67.2 years, grade 1/grade 2: 60/53) were identified from the endocrine clinic. The aetiology of SH include 52 multinodular goitre, 15 Graves’ disease, 7 toxic adenoma and 39 unclassified. A minority of SH patients (5.3 %) progressed to overt hyperthyroidism while 13% remitted to euthyroid state (1 to 3 years with a mean follow-up of 18 months) in the total cohort. Most of the patients remained in SH state during follow-up (50/60 in grade 1 SH and 42/53 in grade SH). However, no single predictive factor could be identified for progression or remission of SH. The prevalence of morbidities in SH subjects include ischaemic heart disease (16.8%), heart failure (8.9%), tachyarrhythmias (13.3%), any cardiovascular disease (28%), cerebrovascular disease (28%), osteoporosis (28%), and any fracture (15.9%). Conclusion: Most of SH cases in our cohort remain in subclinical state with very few progressing to overt hyperthyroidism. Significant proportion of SH subjects have vascular disease, but this association needs to be confirmed in prospective controlled studies.
Subclinical hyperthyroidism (SH) is characterised by low serum concentration of thyrotropin (TSH) in the presence of normal serum thyroid hormones and the absence of obvious symptoms of hyperthyroidism. The reported prevalence of SH is variable (depending on the area, iodine intake, inclusion of exogenous SH), more common in women than in men (ratio of female:male is 1.5: 1) and its incidence increases with advancing age. In the recent Cardiovascular Health Study (CHS) of 3233 American community-dwelling individuals aged 65 years or older, the prevalence of SH (TSH 0.10 to 0.44 mU/L) was 1.5%. The prevalence of thyroid autonomy is inversely correlated with the population’s iodine intake. Singapore has been classified as an iodine-sufficient country by the World Health Organization (WHO) and International Council for the Control of Iodine Deficiency Disorders (ICCIDD). However the exact prevalence of SH in Singapore is currently not known.
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