How is thyrotoxicosis diagnosed?

A diagnosis of thyrotoxicosis can be made when blood tests show that a patient's TSH level is suppressed below the lower limit of the reference range at the same time that thyroid hormone levels are increased above the upper limits of reference ranges. This interpretation is based on idea that the amount of TSH secreted by the pituitary gland is the most sensitive indicator of whether there is an adequate amount of thyroid hormone in the body. Excess thyroid hormone will block secretion of TSH from the pituitary gland, suppressing the TSH level below the lower limit of the reference range. The reference range for TSH is usually listed as 0.5-5.0 mU/ml (microunits per milliliter), though there may be slight variation from laboratory to laboratory. In the setting of suspected thyrotoxicosis, measurement of thyroid hormone is necessary to confirm that a low TSH level is related to excess thyroid hormone. In very rare cases, low TSH levels may be related to disorders that disrupt the normal functioning of the hypothalamus and pituitary gland. A thyroid hormone level that is increased above the upper limit of the reference range provides evidence that a suppressed TSH level reflects the presence of excess thyroid hormone. The most common blood test used to measure the level of thyroid hormone is the T4 test. This test measures the total amount of thyroxine present in the bloodstream. The reference range for T4 is usually listed as 4.5-11.5 mg/ml (micrograms per milliliter). A Free T4 test may also be used to measure the level of thyroid hormone. This test measures the fraction of thyroxine in the bloodstream that is not bound to protein. It may actually provide a more accurate estimate of the level of active thyroid hormone in the body. The reference range for Free T4 is usually listed as 0.8-1.8 ng/ml (nanograms per milliliter), though there may be variation from laboratory to laboratory. In cases where a diagnosis of thyrotoxicosis is suspected but T4 and Free T4 levels are within normal limits, measurement of a T3 test may reveal an increased level of triiodothyronine in the bloodstream. The reference range for T3 is usually listed as 90-200 ng/dl (nanograms per deciliter). A Free T3 test is also available, though it is not used much in practice.