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TSH levels are determined by a classic negative feedback system in which high levels of T3 and T4 suppress the production of TSH, and low levels of T3 and T4 increase the production of TSH. TSH levels are thus often used by doctors as a screening test, where the first approach is to determine whether TSH is elevated, suppressed, or normal. [25]
In overt primary hypothyroidism, TSH levels are high and T 4 and T 3 levels are low. Overt hypothyroidism may also be diagnosed in those who have a TSH on multiple occasions of greater than 5mIU/L, appropriate symptoms, and only a borderline low T 4. [47] It may also be diagnosed in those with a TSH of greater than 10mIU/L. [47]
In sub-clinical hyperthyroidism, serum TSH is abnormally low, but T4- and T3-levels fall within laboratory reference ranges. [47] It primarily affects the skeleton and the cardiovascular system (abnormalities in other systems have also been reported), in a similar but less severe and less frequent way than overt hyperthyroidism does.
It occurs about 7.5 times more often in women than in men. [1] Often, it starts between the ages of 40 and 60, but can begin at any age. [6] It is the most common cause of hyperthyroidism in the United States (about 50 to 80% of cases). [1] [4] The condition is named after Irish surgeon Robert Graves, who described it in 1835. [6]
Therefore, when TRH is given exogenously, TSH levels increase. If the increase in serum TSH level following TRH administration is absent or very slight, then the cause of the hypothyroidism is in the anterior pituitary gland, i.e. the pituitary is not secreting TSH. Therefore, even when TRH is given exogenously, TSH levels do not rise as the ...
A low amount of thyroxine (one of the two thyroid hormones) in the blood, due to lack of dietary iodine to make it, gives rise to high levels of thyroid stimulating hormone (TSH), which stimulates the thyroid gland to increase many biochemical processes; the cellular growth and proliferation can result in the characteristic swelling or hyperplasia of the thyroid gland or goiter.
[8] [12] The hypothalamic-pituitary-thyroid axis is also altered in FHA; TSH levels are low-to-normal and there is an increase in reverse triiodothyronine and low level of triiodothyronine. [3] Relative energy expenditure (REE) is also closely linked to T 3, as is evidenced by a correlation between macronutrient intake, REE, and thyroid hormone ...
Symptoms of hypothyroidism can include low energy, cold intolerance, muscle cramps, constipation, and memory and concentration problems. [38] It is diagnosed by the presence of elevated levels of thyroid stimulation hormone or TSH. Patients with elevated TSH and decreased levels of free thyroxine or T4 are considered to have overt hypothyroidism.
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