Sub clinical hypothyroidism (SCH) is a medical condition, which is present in 3% to 5% of general population. It is more common in women than in men and its prevalence increases with age. Up to 20% of women over the age of 60 years have SCH. After six decades of life, the prevalence in men approaches that in women with a combined prevalence of 10%. The important implication of SCH is that it is likely to proceed to clinical hypothyroidism. It is also a risk factor for cardiovascular disease although according to some, it is debatable. Recent evidence shows that any possibility of increased cardiovascular risk exists in persons younger than 70 years. Those aged 70 to 80 years have no additional risk and those older than 80 years may actually enjoy a protective benefit.
Sub clinical hypothyroidism is considered when serum total thyroid stimulating hormone (TSH) is above its normal limit despite the normal levels of free thyroxine (T4). In 80% of patients with SCH, anti-thyroid antibodies can be detected. 80% of patients with SCH have a serum TSH of less than 10 m IU/L.
The normal ranges of serum total thyroxine (T4) and thyroid stimulating hormone (TSH) are 4.5 to11.5 mcg/dl and 0.3 to 4.70 m IU/L respectively in adults.
- Hashimoto thyroiditis, a common thyroid gland inflammation producing antibodies
- Recent treatment with radioactive iodine for hyperthyroidism
- Treatment with Interferon alfa, an anti-cancer drug
- Treatment with Interluekin-2
- Irregular heart beat treatment with amiodarone
- Treatment with lithium
- Recent pregnancy and child delivery
- Cold intolerance
- Consistent weight gain
- Memory problems
Management of SCH differs whether serum THS level is 3 to 5 m IU/L, 5.1 to 10 m IU/L or higher than 10 m IU/L.
- Serum TSH concentration of 3 to 5 m IU/L—This serum TSH level predisposes a person to a risk of progression to hypothyroidism but no firm evidence of adverse health consequences exists. Normally, no intervention is recommended for this group but a follow-up of serum TSH every year is done.
- Serum THS concentration of 5.1 to 10 m IU/L—According to some experts, the treatment of this group did not show any benefit to the patients. The possibility that the elevated serum TSH level is a cardiovascular risk factor is highly controversial. So the treatment for this group should be individualized depending on the age (favoring it for younger parsons), associated medical conditions, degree of TSH elevation, gradual increase of TSH level, presence of anti-thyroid antibodies, presence of goiter and hypothyroid symptoms.
- Serum THS concentration greater than10 m IU/L—All patients of this group should be treated with levothyroxine even if the thyroxine concentration level is within normal range.
Beginning at age 35, everyone should be screened for thyroid dysfunction every five years. A routine screening for sub-clinical hypothyroidism is not recommended. Serum TSH testing should be obtained in women who have vague suggestive symptoms, who are pregnant or who have a strong family history of auto-immune thyroid disease.
Since SCH can progress to clinical hypothyroidism, it should be managed adequately as per above guidelines. Unnecessary overenthusiastic treatment should be avoided because it has no benefit for the individual.