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subclinical hypothyroidism ( sch ) defined as normal free thyroxine ( t4 ) and elevated thyroid stimulating hormone ( tsh ) is primarily a biochemical diagnosis with or without clinical symptoms .
sch is associated with several long term effects including dyslipidemia , hypertension , subfertility and may be an independent risk factor for cardiovascular morbidity . circulating tsh shows a normal circadian rhythm with a peak between 11 pm-5 am and a nadir between 5 pm-8 pm .
secretory pulses occur every 2 - 3 hours and are interspersed with periods of tonic non - pulsatile tsh secretion . although the tsh secretion is pulsatile , the low amplitude of the pulses and the long half - life of tsh result in only modest circulatory variations .
it is generally observed that tsh in early morning fasting states were higher than tsh levels measured later in the same day . in routine clinical practice not much importance is being given to the timing of the sample or the fasting / non - fasting status of the patient .
however , an entity like sch which heavily relies on tsh values may be under or overdiagnosed based on a single value .
further , in the recent past , narrower and stricter cut - offs for tsh has been advocated for defining euthyroidism in special situations like pregnancy . hence uniformity in testing under standard conditions is necessary . with this background
, we proposed this study to evaluate whether tsh measured in fasting state or postprandially would make a difference .
the study was conducted in the government stanley medical college hospital , chennai , tamilnadu where the thyroid functions are usually done in fasting state only .
fifty - seven adult ambulatory patients were selected from our laboratory database and were divided into group a ( normal freet4 and tsh ) , group b ( sch with increased tsh and normal free t4 ) and group c ( overt hypothyroidism with low free t4 and high tsh ) . the lab reference ranges ( given below ) were used to define low and high values of freet4 and tsh .
the study was approved by the institutional review board , government stanley medical college , chennai and informed consent was obtained prior to phlebotomy from the patients .
phlebotomy was performed after an 8 - 12 hour overnight fast between 7:30 - 8:30 am for free t4 and tsh measurements and the patients returned 2 hours after breakfast for their samples to be rechecked between 10:30 - 11:00 am on the same day .
samples were analyzed by the electrochemiluminesence immunoassay intended for use on eleccsys and cobas immunoassay analyzers .
machine was calibrated and the serum was collected and processed according to manufacturer 's instructions .
the methodology had an analytical sensitivity of 0.005 iu / ml and a functional sensitivity of 0.014 iu / ml ( coeffecient of variation 1.4% )
. suggested normal values for tsh were 0.27 - 4.2 iu / ml and these values correspond to the 2.5 and 97.5% of results obtained from a total of 516 healthy test subjects examined .
suggested normal values for free t4 was 0.80 - 1.8 ng / ml and the values correspond to the 2.5 and 97.5% of results from a total of 801 healthy test subjects studied .
differences in free t4 and tsh levels between fasting and non - fasting state were analyzed by paired student - t test .
differences in free t4 and tsh levels between fasting and non - fasting state were analyzed by paired student - t test .
tsh values were lowered after food when compared to fasting in a statistically significant manner in all the three groups as shown in [ table 1 ] .
fasting and 2 hour post - prandial values ( meanstandard deviation ) of free t4 and tsh among the three groups
in our study we addressed a clinically relevant question : whether thyroid function tests ( free t4 and tsh ) should be estimated in fasting state or not ?
we observed that tsh values get lowered if estimated postprandially irrespective of the fasting levels .
but due to its low pulse amplitude and long half - life the circulating variations is only modest .
previous studies by scobbo et al . , kamat et al . and bandhopadhyay et al .
tsh secretion is heavily dependent on two factors namely thyrotropin releasing hormone ( trh ) and somatostatin ; the former stimulating and the latter inhibiting tsh .
a possible explanation for the acute postprandial decline of serum tsh is food induced elevation of circulating somatostatin and consequent suppression of tsh .
microparticle enzyme immuno assay ( second generation ) , radioimmunoassay , immunofluorescence assay but observed results similar to our study . in a recent study by sarkar comparing two third generation tsh assay methods , [ chemiluminescence ( architect ) vs electrochemiluminescence ( cobas ) ] ,
timing of sampling was considered as one of the factors which might have influenced the decline in tsh in the previous studies .
hence , whether the tsh suppression in our study was due to food related alteration in blood chemistry or timing of sample or both could not be clarified .
clinical guidelines for thyroid function testing or laboratory guidelines for free t4 and tsh estimation do not emphasize the time of phlebotomy or the fasting / non - fasting status of the patient .
clinically , in our study , the lowering of tsh postprandially led to reclassification of 15 out of 20 subjects ( 75% ) as euthyroid who would have otherwise been labelled as sch based on fasting tsh alone .
this may have a significant impact not only on the diagnosis but also monitoring of hypothyroidism , especially in situations where even marginal variations in tsh may be important like in pregnancy or sub - fertility . with recent guidelines for management of hypothyroidism during pregnancy stressing a target tsh of 2.5 miu / l or less
, the findings of our study may have more 0 . further , lack of uniformity in the time of sampling for tsh may lead to unnecessary repetition of tests especially in a resource limited setting . with the above observations
, we propose that a fasting tsh sample may be preferred to random or postprandial estimations as normal fasting values would obviate the need for retesting .
factors other than food or timing of sample have not been addressed in our study .
this may have clinical implications in the diagnosis and management of hypothyroidism , especially sch .
background : thyroid stimulating hormone ( tsh ) levels vary with the time of the day and probably in relation to food . in this study , we addressed the question of whether a fasting or non - fasting sample would make a clinically significant difference in the interpretation of thyroid function tests.materials and methods : fifty seven adult ambulatory patients were selected from our laboratory database and were divided into group a [ normal free thyroxine ( t4 ) and tsh ] , group b ( subclinical hypothyroid with increased tsh and normal free t4 ) and group c ( overt hypothyroid with low free t4 and high tsh ) . 
 thyroid functions ( free t4 and tsh ) were done in fasting state and 2 hours postprandially.results:tsh was suppressed in all subjects after food irrespective of the fasting levels . 
 free t4 values did not change significantly . 
 this resulted in reclassification of 15 out of 20 ( 75% ) subjects as subclinical hypothyroidism ( sch ) based on fasting values whose tsh values were otherwise within range in the postprandial sample . 
 this may have an impact on the diagnosis and management of hypothyroidism especially where even marginal changes in tsh may be clinically relevant as in sch and in pregnancy.conclusion:tsh levels showed a statistically significant decline postprandially in comparison to fasting values . 
 this may have clinical implications in the diagnosis and management of hypothyroidism , especially sch .