Measuring thyroid function in the past was difficult. It was a crude measure of metabolic activity or the total amount of protein bound iodine. Fortunately in the last 20 years, thyroid function testing has become relatively easy with the development of new assays (blood tests that measure the compound in blood) that are very sensitive.
Two basic parameters are used to measure thyroid function; the first parameter is a direct measure of thyroid hormone production (Free thyroxine [FT4] and free tri-iodothyronine [FT3] and the second is the response of the pituitary (TSH) to the level of circulating thyroid hormone. Both these parameters need to be interpreted together in patients with established thyroid disorders.
TSH or thyroid stimulating hormone is the best and most sensitive test. Note that TSH levels go in the reverse direction.
Low TSH means an overactive thyroid; generally a value less than 0.3 mIU/l is abnormal and a value less than 0.01 mIU/l indicates hyperthyroidism.
High TSH means an underactive thyroid; generally a value above 2.5 mIU/l may be considered abnormal and would indicate hypothyroidism.
A normal TSH between 0.3 to 2.5 mIU/l means euthyroidism. Note that different laboratories will quote slightly different normal ranges.
FT4 and FT3 levels are markers of direct thyroid gland and peripheral production of thyroid hormones. These markers measure the unbound fraction (ie the free component) of thyroid hormone that is circulating in the blood stream. Note that most thyroid hormone over 95% is bound to transport proteins and only the free component is available for thyroid hormone action.
FT4 and FT3 are measured at very small quantities and usually measured at picomol/l (pmol/l) levels. A pico is a trillionth. Therefore, if you see a change in FT4 from say 14 to 15 pmol/l, these changes are not significant.
How do I put all this together?
Two basic parameters are used to measure thyroid function; the first parameter is a direct measure of thyroid hormone production (Free thyroxine [FT4] and free tri-iodothyronine [FT3] and the second is the response of the pituitary (TSH) to the level of circulating thyroid hormone. Both these parameters need to be interpreted together in patients with established thyroid disorders.
TSH or thyroid stimulating hormone is the best and most sensitive test. Note that TSH levels go in the reverse direction.
Low TSH means an overactive thyroid; generally a value less than 0.3 mIU/l is abnormal and a value less than 0.01 mIU/l indicates hyperthyroidism.
High TSH means an underactive thyroid; generally a value above 2.5 mIU/l may be considered abnormal and would indicate hypothyroidism.
A normal TSH between 0.3 to 2.5 mIU/l means euthyroidism. Note that different laboratories will quote slightly different normal ranges.
FT4 and FT3 levels are markers of direct thyroid gland and peripheral production of thyroid hormones. These markers measure the unbound fraction (ie the free component) of thyroid hormone that is circulating in the blood stream. Note that most thyroid hormone over 95% is bound to transport proteins and only the free component is available for thyroid hormone action.
FT4 and FT3 are measured at very small quantities and usually measured at picomol/l (pmol/l) levels. A pico is a trillionth. Therefore, if you see a change in FT4 from say 14 to 15 pmol/l, these changes are not significant.
How do I put all this together?
Low TSH, High FT4 and FT3 -----HyperthyroidismHigh TSH, Low FT4 and FT3 -----Hypothyroidism
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