Subclinical hypothyroidism (SCH) is diagnosed when the patient’s blood thyroid level is normal for the testing laboratory and the blood thyroid stimulating hormone (TSH) level is slightly high. The TSH test is more sensitive than the serum thyroxin test. SCH is more common in women, especially with pregnancy or aging, and usually progresses to clinical hypothyroidism.
Most elevations of TSH are the result of autoimmune disease with 80% of SCH patients having antithyroid antibodies. The current normal upper limit of TSH is 5 mIU/L. Many people with levels between 3 and 5 have high levels of thyroid antibodies. There are pros and cons for the lowering of the upper limit of normal for TSH.
It is important to diagnose SCH before it becomes true hypothyroidism. SCH may be a risk factor for cardiovascular disease and can cause abnormal lipid levels. One study showed that SCH in pregnant women was associated with reduced I.Q. in their children. Screening for SCH is most important in women over age 50 years and in pregnant women.
Current studies don’t show significant improvement with levothyroxine treatment. More clinical studies are needed of diagnostic screening and the success rates of treatment.
Currently, the approach is “routine levothyroxine therapy for persons with a persistent serum TSH of more than 10.0 mIU/L and individualized therapy for those with a TSH below 10.0 mIU/L.” Lipid profiles tend to improve with levothyroxine if the TSH is above 10.0 mIU/L. Doses are 50 to 75 micrograms of levothyroxine, daily. TSH level should be repeated after 8 weeks, according to the author. Routine retesting should be done at 6 months and repeated annually, thereafter.
CONCLUSION: If low thyroid activity is suspected and the serum thyroxin levels are normal, a serum TSH level should be done. Levothyroxine treatment is suggested when levels are above 10.0. Diagnosis and treatment below the 10.0 level is controversial. Further studies are needed of the benefits of levothyroxine therapy.
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