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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]
Free T4, total T3 or both are elevated and serum TSH is below normal in hyperthyroidism. If the hyperthyroidism is mild, only serum T3 may be elevated and serum TSH can be low or may not be detected in the blood. [14] Free T4 levels may also be tested in patients who have convincing symptoms of hyper- and hypothyroidism, despite a normal TSH.
In children, late blood sampling at 60 to 120 minutes is necessary. An increase in the serum TSH level following TRH administration means that the cause of the hypothyroidism is in the hypothalamus (tertiary hypothyroidism), i.e. the hypothalamus is not producing TRH. Therefore, when TRH is given exogenously, TSH levels increase.
Hypothyroidism: Reduced thyroid function primary hypothyroidism: Feedback loop interrupted by low thyroid secretory capacity, e.g. after thyroid surgery or in case of autoimmune thyroiditis; secondary hypothyroidism: Feedback loop interrupted on the level of pituitary, e.g. in anterior pituitary failure
The diagnosis of hyperthyroidism is confirmed by blood tests that show a decreased thyroid-stimulating hormone (TSH) level and elevated T 4 and T 3 levels. TSH is a hormone made by the pituitary gland in the brain that tells the thyroid gland how much hormone to make. When there is too much thyroid hormone, the TSH will be low.
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.
Thyroid function tests (TFTs) is a collective term for blood tests used to check the function of the thyroid. [1] TFTs may be requested if a patient is thought to suffer from hyperthyroidism (overactive thyroid) or hypothyroidism (underactive thyroid), or to monitor the effectiveness of either thyroid-suppression or hormone replacement therapy.
They may reveal hyperthyroidism (high T 3 and T 4), hypothyroidism (low T 3, T 4), or subclinical hyperthyroidism (normal T 3 and T 4 with a low TSH). [84] TSH levels are considered the most sensitive marker of thyroid dysfunction. [84]