TSH can be falsely decreased by medications including dexamethasone and corticosteroids. When possible, discontinue medications for several days or more prior to testing thyroid levels. Ideally, measure TSH in the morning (between 8 and 9am) and fast overnight (TSH levels are affected eating food high in fiber).
The researchers of the study "Clinical Significance of TSH Circadian Variability in Patients with Hypothyroidism" (Sviridonova MA, et. al.) found an average morning TSH level in the subclinically hypothyroid group was 5.83 mU/L and in the afternoon was 3.79 mU/L; 73% TSH circadian variability. Morning (8 - 9am), Afternoon (2 - 4pm).
In addition, I wanted to point out that TSH is a pituitary hormone that is totally inadequate as the primary diagnostic for thyroid issues. At best TSH is an indicator, to be considered along with more important indicators such as symptoms, and also levels of the biologically active thyroid hormones, Free T3 and Free T4 (not the same as Total T3 and T4).
A good thyroid doctor will treat a hypo patient clinically by testing and adjusting Free T3 and Free T4 as necessary to relieve symptoms, without being constrained by resultant TSH levels. You can get some good insight into clinical treatment from this letter written by a good thyroid doctor for patients that he sometimes consults with after initial tests and evaluation. The letter is then sent to the participating doctor of the patient to help guide treatment. In the letter, please note the statement, "the ultimate
criterion for dose adjustment must always be the clinical response of the patient."
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