Thought some people might be interested in this new paper on PubMed:
This large study showed the variation in the reference ranges with age, sex, ethnicity, time of day and month of year.
Some excerpts I found interesting from within the article:
1. "To avoid incorrectly identifying TSH values as abnormal, the TSH reference range needs to take into account the subject’s age and the time of day when the blood sample was drawn. For example, a TSH value of 7.5mIU/L would generally be considered elevated and indicative of subclinical hypothyroidism. However, if the sample was drawn from an 80 year old woman at midnight, a TSH of 7.5 mIU/L is within the 95% reference range and is in fact normal."
2. "TSH suppression is standard treatment of thyroid cancer (57,58), and documenting TSH suppression is based upon a single TSH measurement. The measurement is usually obtained between the hours of 0700 and 2000, when TSH is normally lowest. TSH suppression during the day does not invariably mean that TSH is suppressed at night. Studies to determine the relationship between daytime TSH suppression and nighttime TSH sup- pression and the relationship between partial and complete 24 hour TSH suppression on thyroid cancer need to be performed. Determining TSH suppression in patients with thyroid cancer from a single daytime TSH value is similar to determining glycemic control in an individual with diabetes based upon a single blood glucose measurement. Selection bias makes the interpretation of the results and their clinical relevance questionable."
3. "The study assumes that immunoactivity corresponds with bioactivity. This is not necessarily the case. Thyrotroph cells secrete a variety of isoforms of TSH and immunoreactivity of the isoforms does not parallel bioactivity. Additionally, isoform variation can affect immunoassay results (61). The nocturnal TSH surge may reflect a variation in TSH isoform secretion from the pituitary (58) and not necessarily a change the concentration of bioactive TSH (61–63). The nocturnal rise in T3, however, suggests that the nocturnal TSH rise does in fact have target tissue effects (22)."
More about TSH isoforms here:
Turns out the standard TSH immunoassay tests determine a total concentration of 'TSH' made up of various 'isoforms' or chemical and structural variants of TSH which all have the same immunoreactivity. But the different isoforms can have different biological activity in the sense that they have different effects on thyroid tissue, hormone production etc. So the measured 'TSH' may not correspond to the effective TSH activity in the body.
The human body gets more complicated with every article I read...