Time of day makes a difference in thyroid testing
Jacob Schor ND
August 14, 2012
The level of the hormone, TSH that is often used to evaluate thyroid function, varies with the time of day the samples are collected. Collecting afternoon samples can lead to under diagnosis of hypothyroidism.
Doctors test thyroid stimulating hormone, also known as TSH, to screen for and diagnose hypothyroidism, a condition of inadequate thyroid hormone. TSH is made in the brain and as its name implies, stimulates the thyroid gland to produce more thyroid hormone. Thus if TSH levels increase, it the brain is telling the body to make more thyroid hormone. When TSH levels stay high it suggests that the body cannot make enough thyroid hormone. Most laboratories define ‘normal’ TSH as between 0.5 and 5.0 uM/L. TSH levels above 5.0 are considered hypothyroidism and those below 0.5, hyperthyroidism.
A new study published in the August 2012 issue of Endocrine Research reveals that our current approach to diagnosing hypothyroidism has been too simplistic and needs change. Production of TSH follows a circadian rhythm that is it follows a 24-hour cycle with regular fluctuations. TSH production peaks at its highest level between 2:00 and 4:00 am each morning and then drops reaching its low point between 4 and 8 pm in the evening. Though this phenomenon is well known the actual extent of these fluctuations was much greater than was previously thought.
In this current study, researchers from the Federal Endocrinological Research Centre in Moscow, tested 20 women with subclinical hypothyroidism. Morning and afternoon TSH levels were measured and compared. The median morning TSH value for women with subclinical hypothyroidism was 5.83 mU/L. Their afternoon median TSH dropped down to 3.79 mU/L. Following the current guidelines for diagnosis hypothyroidism would have been diagnosed in only half of these cases when tested in the afternoon.
These new data explain why many people who appear to have symptoms of hypothyroidism have been told their test results were normal, that they are fine, when in truth they hypothyroid. TSH levels measured in blood samples drawn in the afternoon should be viewed with caution, as they may be falsely low and appear normal. Samples for all thyroid tests should be collected first thing in the morning.
It gets more complicated than this. Many practitioners consider the current normal range for TSH values to be too broad. Rather than judging TSH levels above 5 as hypothryoidism, some consider the upper limit of normal to be as low as 3.0, while researchers looking at what is called sub-clinical hypothyroidism define the condition by a TSH greater than 2.5. Apparently that is the level at which risk for cardiovascular disease increases significantly.
Combining these two bits of news, the circadian variability of TSH and the benefits of treating even slight increases in TSH, leads me to think that we should be relooking at a fair number of patients who have previously been told that their thyroids were fine. They might not be…..
Sviridonova MA, Fadeyev VV, Sych YP, Melnichenko GA. Clinical Significance of TSH Circadian Variability in Patients with Hypothyroidism. Endocr Res. 2012 Aug 2.
Duntas LH, Biondi B. New insights into subclinical hypothyroidism and cardiovascular risk. Semin Thromb Hemost. 2011 Feb;37(1):27-34. Epub 2011 Jan 19.