Lets start with . . Prevalence of both disorders increase with age
Definition is a laboratory diagnosis.
To review Not bound to thyroxine-binding globulin (TBG), free fraction responsible for hormone actions. T4 is converted to T3 in the periphery, the more potent form of the hormone.
Think of thyroid dysfunction as a spectrum
Incredibly common Well-known causal relationship between hyperthyroidism and AF, though significance in subclinical disease unknown
The article I am presenting is the Gammage et al study from the May 14 th 2007 issue of Archives of Internal Medicine. Gammage MD, Parle JV, Holder RL, Roberts LM, Hobbs FD, Wilson S, Sheppard MC, Franklyn JA. Association Between Serum Free Thyroxine Concentration and Atrial Fibrillation. Arch Intern Med . 2007;167:928-934. Take framingham RFs and see if independent predictors in this cohort
Took all subjects and measured TSH, FT4, and FT3, did an ECGm and reviewed their chart for meds and RFs
Results: The distribution of subjects into the classes of thyroid dysfunction has the greatest number in the euthyroid category.
Based on thyroid status, the prevalence of AF is shown here. You can see that the euthyroid group has similar prevalence to the cohort as a whole, but the subclinical hyperthyroid group . . .
The novel finding in this study relates to AF and free T4. Surprisingly, TSH was not After logistic regression analysis of presence of AF vs risk factors for AF, AF and free T4 still significantly associated IQR – interquartile range = range covered by middle half of data Q3 – Q1
This shows the relationship between AF prevalence and free T4 graphically
I felt the strengths of this study include their efforts to decrease bias.
What do these findings tell us about screening elderly populations for mild thyroid disease and should we treat it?
This leads us to needed future studies . . .
Subclinical thyroid disease and high normal free T4 predict presence of AF Rachel H. Kon, M.D. GIM Journal Club July 24,2007
All read by a single cardiologist blinded to thyroid status and patient details
Patient survey and chart review to identify all current drug treatments and major risk factors for AF the subjects had before study
*11 subjects did not fit into one of these 5 diagnostic categories and were excluded from analyses based on thyroid status Thyroid Status Subjects Overt Hyperthyroid 14 Subclinical Hyperthyroid 126 Euthyroid 5519 Subclinical Hypothyroid 167 Overt Hypothyroid 23 Total 5849
279 cases of AF in cohort: 150 cases of AF newly dx in study, 129 were previously dx *Table adapted from Gammage et al table 1 Significantly higher prevalence of AF in subclinical hyperthyroid vs. Euthyroid group and whole cohort (p=0.01) AF more prevalent in males vs females (p<0.001) Thyroid Status Whole Cohort Male Female Overt Hyperthyroid 0% 0% 0% Subclinical Hyperthyroid 9.5% 11.1% 8.3% Euthyroid 4.7% 6.5% 2.9% Subclinical Hypothyroid 4.2% 6.8% 2.8% Overt Hypothyroid 0% 0% 0% All Subjects 4.8% 6.6% 3.1%
Significantly higher free T4 in AF patients (p< 0.001) No significant difference in TSH concentration between patients with and without AF When exclude subjects with previously undiagnosed overt hyperthyroidism and hypothyroidism , still independent predictor of AF (p<0.001) Increased serum free T4 is an independent predictor of AF (p=0.004) Normal Range for free T4: 0.70 – 1.55 ng/dL When excluded subjects taking amiodarone, still independent predictor of AF (p=0.01) Whole Cohort With AF Without AF Median free T4 1.14 ng/dL 1.10 ng/dL IQR 0.12-1.27 ng/dL 1.00-1.22 ng/dL
Higher prevalence of AF in patients with higher free T4 concentrations
Gammage MD, Parle JV, Holder RL, Roberts LM, Hobbs FD, Wilson S, Sheppard MC, Franklyn JA. Association Between Serum Free Thyroxine Concentration and Atrial Fibrillation. Arch Intern Med . 2007;167:928-934.
Biondi B et al. Subclinical hyperthyroidism: clinical features and treatment options. European Journal of Endocrinology 2005; 152:1-9.
Biondi et al, Endogenous subclinical hyperthyroidism affects quality of life and cardiac morphology and function in young and middle-aged patients. J Clin Endocrinol Metab. 2000;85:4701-4705.
Cappola et al, Thyroid status, cardiovascular risk, and mortality in older adults. JAMA. 2006;295:1033-1041.
Sawin et al, Low serum thyrotropin concentrations as a risk factor for atrial fibrillation in older persons. N Engl J Med. 1994;331:1249-1252.