Subclinical hypothyroidism, a milder form of hypothyroidism characterized by normal thyroxine levels and an elevated TSH level, is thought to occur in 4.3–8.5% of people in the United States. Hypothyroidism is more common in women than in men. People over the age of 60 are more commonly affected. Dogs are also known to develop hypothyroidism, as are cats and horses, albeit more rarely. The word hypothyroidism is from  Greek hypo- reduced,  thyreos shield, and eidos form. Signs and symptoms
People with hypothyroidism often have no or only mild symptoms. Numerous symptoms and signs are associated with hypothyroidism and can be related to the underlying cause, or a direct effect of having not enough thyroid hormones. Hashimotos thyroiditis may present with the mass effect of a goiter (enlarged thyroid gland). In middle-aged women, the symptoms may be mistaken for those of menopause. Delayed relaxation after testing the ankle jerk reflex is a characteristic sign of hypothyroidism and is associated with the severity of the hormone deficit. Myxedema coma
Myxedema coma is a rare but life-threatening state of extreme hypothyroidism. It may occur in those with established hypothyroidism when they develop an acute illness. Myxedema coma can be the first presentation of hypothyroidism. People with myxedema coma typically have a low body temperature without shivering, confusion, a slow heart rate and reduced breathing effort. There may be physical signs suggestive of hypothyroidism, such as skin changes or enlargement of the tongue. Pregnancy
Even mild or subclinical hypothyroidism leads to possible infertility and an increased risk of miscarriage.
It also contains calcitonin (a hormone produced in the thyroid gland involved in the regulation of calcium levels), T1 and T2; these are not present in synthetic hormone medication. This extract was once a mainstream hypothyroidism treatment, but its use today is unsupported by evidence; British Thyroid Association and American professional guidelines discourage its use, as does NICE. Subclinical hypothyroidism
There is no evidence of a benefit from treating subclinical hypothyroidism in those who are not pregnant, and there are potential risks of overtreatment. Untreated subclinical hypothyroidism may be associated with a modest increase in the risk of coronary artery disease when the TSH is over 10 mIU/L. A 2007 review found no benefit of thyroid hormone replacement except for "some parameters of lipid profiles and left ventricular function". There is no association between subclinical hypothyroidism and an increased risk of bone fractures, nor is there a link with cognitive decline.Since 2008, consensus American opinion has been that in general people with TSH under 10 to 20 mIU/L do not require treatment.American guidelines recommend that treatment should be considered in people with symptoms of hypothyroidism, detectable antibodies against thyroid peroxidase, a history of heart disease or are at an increased risk for heart disease, if the TSH is elevated but below 10 mIU/L. NICE recommends that those with a TSH above 10 mIU/L should be treated in the same way as overt hypothyroidism.