Is there a need to redefine the upper normal limit of TSH?

Abstract

Mild forms of hypothyroidism -subclinical hypothyroidism -have recently been discussed as being a risk factor for the development of overt thyroid dysfunction and for a number of clinical disorders. The diagnosis critically depends on the definition of the upper normal limit of serum TSH as, by definition, free thyroxine serum concentrations are normal. Cut-off levels of 4-5 mU TSH/l have been conventionally used to diagnose an elevated TSH serum concentration. Recent data from large population studies have suggested a much lower TSH cut-off with an upper limit of 2-2.5 mU/l but application of strict criteria for inclusion of subjects from the general population studies aiming at assessing TSH reference intervals (no personal or family history of thyroid disease, no thyroid antibodies and a normal thyroid on ultrasonography) did not result in an unequivocal upper limit of normal TSH at 2.0 -2.5 mU/l. When summarizing the available evidence for lowered upper TSH cut-off values and their potential therapeutic implications there is presently insufficient justification to lower the upper normal limit of TSH and, for practical purposes, it is still recommended to maintain the TSH reference interval of 0.4-4.0 mU/l. Classifying subjects with a TSH value between 2 and 4 mU/l as abnormal, as well as intervening with thyroxine treatment in such subjects, is probably doing more harm than good.European Journal of Endocrinology 154 633-637

Determination of the Effect of the Elimination Diet Applied for Overweight and Obese People with Food Intolerance on Body Composition and Biochemical Parameters

“…Free thyroxine (F-T4), free triiodothyronine (F-T3), and thyroid stimulating hormone (TSH) and insulin were measured on cobas e 601 module (Electrochemiluminescence Technology, Roche Diagnostics), while the remaining analytes on cobas c 501 clinical chemistry module (Photometric Technology), according to the instructions of the manufacturer. TSH was considered normal if it was between 0.4 and 4.0 mIU/L [ 28 ]. The reference intervals for FT3 and FT4 were 2.70-5.20 pg/mL and 0.80-1.90 ng/dL, respectively These biochemical parameters are routine tests for the patients that are followed-up and treated at the YorkTest Turkey Laboratory.…”

Serum thyrotropin and free thyroxine reference ranges as defined in a disease-free sample of French middle-aged adults

“…This suggestion raised a storm of controversy (29)(30)(31). Despite the fact that examination of individuals was not based on NACB guidelines, which was not published when data were collected for our study, our interval limits can be assumed to be correct for middle-aged French adults.…”

Decreased serum TSH levels are not associated with mortality in the adult northeast German population

“…Another limitation of our study is the less well-standardized time of blood specimen collection. It has been previously reported that serum TSH levels might alter by daytime (35). Therefore, we performed analyses stratified by the time of blood sampling, which did not change the results materially.…”