The prevalence of subclinical hypothyroidism (SH) is 3-10%. The prevalence of subclinical hyperthyroidism
(SHr) is 0.7-9.7%. Thyroid hormones affect cardiac electrophysiology, contractility, and vasculature.
SH is associated with an increased risk of coronary heart disease (CHD), especially in subjects under 65.
SHr seems to be associated with a slightly increased risk of CHD and an increase in CHD-related mortality.
Both SH and SHr carry an increased risk of developing heart failure (HF), especially in those under 65. Both
SH and SHr are associated with worse prognoses in patients with existing HF. SH is probably not associated
with atrial fibrillation (AF). SHr, low normal thyroid-stimulating hormone (TSH) and high normal free thyroxine
(FT4) are all associated with the increased risk of AF. An association between endothelial dysfunction
and SH seems to exist. Data regarding the influence of SHr on the peripheral vascular system are conflicting.
SH is a risk factor for stroke in subjects under 65. SHr does not increase the risk of stroke. Both SH and SHr
have an unfavourable effect on cardiovascular disease (CVD) and all-cause mortality. There is a U-shaped
curve of mortality in relation to TSH concentrations. A major factor that modifies the relation between subclinical
thyroid disease (SCTD) and mortality is age. SH increases blood pressure (BP). SHr has no significant effect
on BP. Lipids are increased in patients with SH. In SHr, high-density lipoprotein cholesterol and lipoprotein(
a) are increased. SCTD should be treated when TSH is over 10 mU/l or under 0.1 mU/l. Treatment indications
are less clear when TSH is between normal limits and 0.1 or 10 mU/L. The current state of knowledge supports
the understanding of SCTD’s role as a risk factor for CVD development. Age is a significant confounding
factor, probably due to age-associated changes in the TSH reference levels.