Sample Timing, diagnosis of Subclinical Thyroid Dysfunction and Mortality in Acute Myocardial Infarction: ThyrAMI1 study.

OBJECTIVE

The objective of this study was to determine the impact of blood sample timing on the diagnosis of subclinical thyroid dysfunction (SCTD) and mortality in patients with acute myocardial infarction (AMI).

PATIENTS, DESIGN AND MAIN OUTCOME MEASURES

Patients with AMI had thyroid function evaluated on admission between December 2014 and December 2016 and those with abnormal serum TSH had repeat thyroid function assessed at least a week later. The association between sample timing and SCTD was evaluated by logistic regression analysis. Secondary outcomes were confirmation of SCTD on repeat testing and all-cause mortality up to June 2018.

RESULTS

Of the 1806 patients [29.2% women, mean (±SD) age of 64.2 (±12.1) years] analysed, the prevalence of subclinical hypothyroidism (SCH) was 17.2% (n=311) and subclinical hyperthyroidism (SHyper) was 1.2% (n=22) using a uniform TSH reference interval. The risk of being diagnosed with SCTD varied by sample timing in fully-adjusted models. The risk of SCH was highest between 00:01-06:00hrs and lowest between 12:01-18:00hrs, p for trend <0.001, and risk of SHyper was highest between 12:01-18:00hrs and lowest between 00:01-06:00hrs. Furthermore, time of the initial sample was associated with the risk of remaining in a SCH state subsequently. Mortality in SCH patients was not elevated when a uniform TSH reference interval was utilised. However, when time-period-specific TSH reference ranges were utilised, the mortality risk was significantly higher in SCH patients with HR (95% CI) of 2.26 (1.01-5.19), p=0.04.

CONCLUSIONS

Sample timing impacts on the diagnosis and prognosis of SCH in AMI patients. If sample timing is not accounted for, SCH is systemically misclassified, and its measurable influence on mortality is lost.