Common process.
Prevalence increases with age.
Subclinical hypothyroidism is defined is elevated thyrotropin in combination with normal range free thyroxine levels.
Mild thyroid failure due to autoimmune thyroiditis is the most common cause of mildly elevated serum thyrotropin levels.
With overt hypothyroidism thyrotropin, TSH, levels are appropriately elevated, while in subclinical hypothyroidism serum thyroid hormone levels are within the range of normal, but serum thyrotropin levels are elevated outside the reference range.
Elevated serum thyrotrophin levels is likely a normal consequence of aging.
Diagnosis of subclinical hypothyroidism is a biochemical one, solely based on thyroid function testing.
Patient was subclinical hypothyroidism aged 80 years and older treated with levothyroxine, compared with placebo, was not significantly associated with improvement in hypothyroid symptoms or fatigue (Mooijaart S).
Although elevated TSH levels a characteristic of primary thyroid failure, other processes such as external radiotherapy to the neck, drugs such as lithium, or laboratory analomies, heterophilic antibodies in the serum may result in this elevated serum thyrotropin levels.
Subclinical hypothyroidism association with mortality is well-established in young individuals.
The prevalence of subclinical hypothyroidism is higher in women and in the elderly.
In iodine sufficient populations, subclinical hypothyroidism affects of the 10% of the population, with the highest prevalence in women in elderly person.
Subclinical hypothyroidism frequently reverts to euthyroidism and thyrotropin levels rise as people without thyroid disease age, making it likely at the prevalence of subclinical hypothyroidism has been overestimated.
In subclinical hypothyroidism, an intrinsic thyroid disease or inflammation thyroid hormone output does not appropriately increase in response to an elevated serum thyrotropin, leading to a chronically elevated thyrotropin level.
Patients with the thyroid function test suggesting subclinical hypothyroidism do not experience thyroid disease related symptoms more often than euthyroid individuals.
In subclinical hypothyroidism focus should be on concomitant diseases rather than expecting symptomatic relief following level thyroxine substitution.
In individuals aged more than 65 years, the association between subclinical hypothyroidism and ischemic heart disease, or mortality is less convincing.
Many studies have failed to find an association between subclinical hypothyroidism and mortality in individuals older than 65 years while others found an association with thyroid stimulating hormone value is greater than 10 mIU/L.
An estimated 13 million people have subclinical hypothyroidism in the US.
Subclinical hypothyroidism is often treated with thyroid hormones, particularly when it occurs with symptoms attributable to hypothyroidism, such as tiredness constipation and unexplained weight gain.
Thyroid hormone therapy in subclinical hypothyroidism is not associated with improvements in the quality of life based on a metaanalysis of 21 randomized trials: findings do not support the routine use of thyroid hormone in adults with subclinical hypothyroidism (Feller M).
Subclinical hypothyroidism is a common in the general population.
Subclinical hypothyroidism are classified into two groups: those with mildly elevated TSH (4.5-10 mU/l) and those with more marked TSH elevation (TSH >10 mU/l).
The prevalence of subclinical hypothyroidism is around 5-10% in the general population.
Subclinical hypothyroidism is more common in women and increases with increasing age.
Subclinical hypothyroidism is more common in white than in black populations.
TSH does not vary with age in men.
TSH levels markedly increase in women aged more than 45 years.
The major causes for subclinical hypothyroidism are autoimmune thyroiditis (Hashimoto’s disease) and previous treatment for hyperthyroidism.
Treatment of hyperthyroidism with radioiodine results in hypothyroidism in at least 50% of patients.
Partial thyroidectomy is associated with a risk of development of hypothyroidism.
Subclinical hypothyroidism may be a transient phenomenon.
Subclinical hypothyroidism does not always progress to thyroid failure.
The development of hypothyroidism occurs in 51/2-20% of patients with Graves’ disease.
In party treated with thyroxine for hypothyroidism, a high serum TSH indicating that the dose prescribed is inadequate or compliance poor.
Other groups at particular risk of subclinical hypothyroidism include those patients at risk for subclinical hypothyroidism with autoimmune diseases and type I diabetes mellitus and Addison’s disease.
Down’s and Turner’s syndromes are both associated with the development of both subclinical and overt thyroid failure of autoimmune etiology.
The risk of subclinical hypothyroidism during pregnancy is higher in women identified in the first trimester as having positive antithyroid antibodies.
Antithyroid antibodies are risk factors for the development of post-partum thyroiditis, as subclinical or overt hypothyroidism being a feature of postpartum thyroiditis in about 75% of cases.
Radiotherapy to the head and neck is a cause of subclinical hypothyroidism.
Non-thyroidal’ illness may be associated with a transient increase in serum TSH.
Drugs such as lithium and amiodarone can induce subclinical hypothyroidism.
The administration of iodine containing compounds such as radiographic contrast agents is associated with subclinical hypothyroidism.
With modest elevation of serum TSH (5.5-10.0 mU/l), the TSH measurement returns spontaneously to the reference range in more than 60% of cases during 5 years of follow-up.
Subclinical hypothyroidism, has no association with tests of cognitive function, anxiety or depression.
Overt hypothyroidism results in increases in total and low-density lipoprotein (LDL) cholesterol, as well as changes in other lipoprotein and apolipoprotein concentrations but lipid changes in subclinical hypothyroidism are considerably less marked and the results of studies are inconsistent.
In a study of 1200 subjects aged more than 60 years followed for 10 years, there was no association of subclinical hypothyroidism with circulatory mortality.
In a Leiden study of those aged more than 85 years, raised TSH was associated with increased longevity and decreased risk of death from cardiovascular disease.
The longitudinal Cardiovascular Health Study in the United States found no association between subclinical hypothyroidism and the incidences of cardiovascular or cerebrovascular diseases, nor with all-cause mortality.
Recent studies have found associations of subclinical hypothyroidism with cardiovascular disease morbidity and mortality, although others have not.
A meta-analysis of individual participant data from 11 prospective cohort studies has shown no overall association of subclinical hypothyroidism with coronary heart disease events, mortality or total mortality, but significant associations when the degree of elevation of serum TSH was stratified: coronary heart disease events and mortality risks were significantly increased when analysis was confined to those with serum TSH>10 mU/l.
Incident heart failure risk is evident or is of greater magnitude, when serum TSH is >10 mU/l.
Thyroxine treatment has been found to not improve symptoms or mood, unless serum TSH is >10 mU/l.
Systolic and diastolic heart function did not differ after treatment with levothyroxine compared with placebo in older adults with mild subclinical hypothyroidism. (Gencer B).
Improvements in systolic and diastolic function as well as endothelial function and carotid intima-media thickness have all been described with thyroxine treatment in subclinical hypothyroidism.
Thyroxine treatment of subclinical hypothyroidism was associated with lower heart failure risk and lower all-cause mortality in two studies.
Miscarriage rates and rates or premature delivery are lower if subclinical hypothyroidism is treated with thyroxine.
Mild thyroid hormone deficiency is associated with an adverse effect on childhood neurodevelopment, has led to the support the role of thyroxine treatment.
Outside the context of pregnancy, there is considerable debate as to the role of thyroxine therapy in subclinical hypothyroidism, especially in mild cases.
In patients with serum TSH values >10 mU/l and adverse findings the consensus view is in support of treatment with thyroxine in this group.
In general, sub clinical hypothyroidism is not a clinical entity that requires thyroid replacement therapy, and relief of symptoms and some patients may be the result of placebo effect.Subclinical hypothyroidism
There is insufficient evidence to warrant treatment of those with mildly elevated TSH.