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While screening patients for thyroid disease, physicians often find increased thyrotropin-stimulating hormone (TSH) levels in patients whose free thyroxine (T 4 ) levels are not below normal. This is known as Subclinical hypothyroidism.
Measurement of serum TSH is generally considered the best screening test for thyroid disease; increased values usually indicate hypothyroidism, and decreased values usually indicate hyperthyroidism. This test has proved to be both sensitive and specific.
It is common in older people.
One study found 7 % of women and 3% of men aged 60-89 years had TSH level greater than 10 mU/l. Normal range 0.5-4.5mU/l.Range in men is 2-8%. In women 5-10%.
Women with subclinical hyperthyroidismalso had a greater incidence of myocardial infarction(odds ratio, 2.3; 95% CI, 1.3 to 4.0).
The presence of antibodies to thyroid peroxidase (anindicator of thyroid autoimmunity) did not alone predict agreater risk of atherosclerosis or myocardial infarction.
However, the combination of subclinical hypothyroidism and thepresence of these antibodies carried a greater risk of cardio-vascular disease than did subclinical hypothyroidism alone.
Causes of Subclinical hypothyroidism
The term 'subclinical' may be inaccurate, as some of the patients have symptoms. Causes are the same as those of overt thyroid disease.
The main cause is chronic, autoimmune thyroiditis - Hashimoto's disease, Graves' disease.
Thyroid hormone resistance
Thyroid hormone resistance is an inherited condition caused by an abnormality of the thyroid hormone receptor. Mutations to the receptor result in the need for higher levels of thyroid hormones to achieve the same intracellular effect.
As a result, the normal feedback control mechanisms result in high blood levels of thyroxine with a normal TSH in order to maintain a euthyroid state.
Thyroid storm
Hypothyroidism
We must avoid driving TSH levels too low, since this can produce subclinical hyperthyroidism, a condition associated with cardiac arrhythmias. 1-8 In addition, studies 9-11 have shown a decrease in bone mineral density-and therefore an increased risk of osteoporosis-in patients with subclinical hyperthyroidism.
Because osteoporosis, cardiac arrhythmias, and diastolic dysfunction can increase mortality, thyroid hormone replacement should aim for TSH levels at or above the low-normal range.
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