8/3/2023 0 Comments Normal tsh levelsHence, 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. Timing of sampling was considered as one of the factors which might have influenced the decline in TSH in the previous studies. In a recent study by Sarkar comparing two third generation TSH assay methods,, the inter assay variations were well within the limits of agreement. Microparticle Enzyme Immuno Assay (second generation), Radioimmunoassay, immunofluorescence assay but observed results similar to our study. The three previous studies addressing this issue have used different assays for TSH viz. Further, the TSH variation is unlikely to have been due to assay differences. A possible explanation for the acute postprandial decline of serum TSH is food induced elevation of circulating somatostatin and consequent suppression of TSH. TSH secretion is heavily dependent on two factors namely Thyrotropin Releasing Hormone (TRH) and somatostatin the former stimulating and the latter inhibiting TSH. have shown postprandial TSH decline similar to our study. Previous studies by Scobbo et al., Kamat et al. But due to its low pulse amplitude and long half-life the circulating variations is only modest. TSH is a glycoprotein hormone secreted in a pulsatile fashion. The reason for the above observation is not clear. 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. 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. Suggested normal values for TSH were 0.27-4.2 μIU/ml and these values correspond to the 2.5 and 97.5% of results obtained from a total of 516 healthy test subjects examined. The methodology had an analytical sensitivity of 0.005 μIU/ml and a functional sensitivity of 0.014 μIU/ml (coeffecient of variation 1.4%). Machine was calibrated and the serum was collected and processed according to manufacturer's instructions. Samples were analyzed by the Electrochemiluminesence immunoassay intended for use on Eleccsys and Cobas immunoassay analyzers. 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. 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. Patients with renal or liver dysfunction, steroid or thyroxine therapy were excluded. The lab reference ranges (given below) were used to define low and high values of freeT4 and TSH. 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 study was conducted in the Government Stanley Medical College Hospital, Chennai, Tamilnadu where the thyroid functions are usually done in fasting state only. With this background, we proposed this study to evaluate whether TSH measured in fasting state or postprandially would make a difference. Hence uniformity in testing under standard conditions is necessary. Further, in the recent past, narrower and stricter cut-offs for TSH has been advocated for defining euthyroidism in special situations like pregnancy. However, an entity like SCH which heavily relies on TSH values may be under or overdiagnosed based on a single value. 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. It is generally observed that TSH in early morning fasting states were higher than TSH levels measured later in the same day. 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. Secretory pulses occur every 2-3 hours and are interspersed with periods of tonic non-pulsatile TSH secretion. Circulating TSH shows a normal circadian rhythm with a peak between 11 pm-5 am and a nadir between 5 pm-8 pm. SCH is associated with several long term effects including dyslipidemia, hypertension, subfertility and may be an independent risk factor for cardiovascular morbidity. 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. Hypothyroidism is commonly encountered in clinical practice.
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