4.2 Comparison with previous studies
Some population databases have shown an increased cardiovascular risk in mild SCH. In Australia, and a reanalysis of the cohort Whickham Survey in Great Britain, an increase was found in both fatal and nonfatal ischemic cardiovascular disease.11,22 In Taiwan, an increase was shown in only fatal cardiovascular disease.12 A study evaluated the correlation between mild SCH and scores on the Framingham scale for predicting cardiovascular risk. A significant positive correlation was found only in the female population.23 The limitations of the studies mentioned include the fact that due to their basis in population databases, these are no studies designed to evaluate the association. Another limitation is that populations with cardiovascular risk factors were included, and although the results were adjusted, possible confounders may remain.
Other studies have evaluated the association of SCH with atherosclerosis markers. Masaaki et al. evaluated the pulse wave velocity in mild SCH in comparison to controls. They found greater pulse wave velocity in mild SCH, indicating a higher degree of atherosclerosis and endothelial stiffness. One difference with our study is that the mean population age was higher (65.2 ± 0.57 years).9 Another marker for atherosclerosis that has suggested an increase in cardiovascular risk in SCH is the reduction in flow-mediated dilation. The mean of TSH was 12.5 ± 8.6 µIU/mL, higher than in our study.16
Unal et al. evaluated the carotid intima-media thickness in children with mild SCH (age 8.1 ± 3.6 years). As with our study, the intima-media thickness was greater with SCH in comparison with HC (0.5 ± 0.19 mm and 0.4 ± 0.0 mm, respectively, p=0.001). In children with mild SCH, they also observed an increase in total cholesterol, LDL, total cholesterol/HDL, and LDL/HDL. A limitation of this study is that there was no regression analysis performed to confirm the significance of SCH as an independent variable in carotid intima-media thickness.24 A difference with our study is that we did not obtain a significant difference in the level of lipids in SCH in comparison to HC.
A study evaluated the association of mild SCH with nonfatal ischemic cardiac disease in postmenopausal women, with no association found. The age of the population was higher than in our study, with an average of > 60 years.13 Rhee et al. evaluated the association between mild SCH and fatal ischemic cardiac disease in populations with and without cardiac insufficiency. No association was demonstrated in the population with a history of cardiac insufficiency (population with greater cardiovascular risk). The mean age of the population was higher (59.2 ± 19.2 years) than in our study.14 Selmer et al. found a slight reduction in mortality by all causes in patients with mild SCH in a population with an average of <60 years.25 One of the limitations of the aforementioned studies is that they were based on population databases, without a design established to evaluate the association. Another limitation is that populations with cardiovascular risk factors were included, limiting the results of the association.13,14,25
Treatment is currently recommended in SCH with TSH >10 µIU/mL. Treatment in mild SCH is recommended only in populations with goiter, pregnancies, progression in TSH levels, and/or symptoms suggesting hypothyroidism with positive anti-TPO. However, the symptoms of hypothyroidism can be subjective.10
As in this, other studies have shown an improvement in the cardiovascular system following the use of levothyroxine in mild SCH. Razvi et al. found a reduction in fatal and nonfatal ischemic cardiac disease. The population age range was 40 to 70 years. However, in the subgroup with ages >70 years, no significant changes were observed.26 Also, the change in flow-mediated dilation with the use of levothyroxine was evaluated. An increase in this variable was obtained three months after treatment began, which suggests an improvement in endothelial function. The mean age of the population was <60 years,27 as was the case in our study. An improvement in cardiac contractility in a population with a mean age <40 years was also observed.28,29
In our study, in the three baseline groups, we obtained a weak positive correlation between carotid intima-media thickness and the anti-TPO level. A study evaluated the anti-TPO level with the presence of atherosclerosis in cardiac catheterization. No association was found.30 Wells et al. also did not find a difference in cardiovascular risk factors in populations with or without anti-TPO.29
In terms of the carotid intima-media, thickening is considered a measurement of >0.9 mm. 31,32 In our study, the mean carotid intima-media thickness was <0.9 mm in the three baseline populations. Despite the foregoing, significant differences were found among the groups. In a study that evaluated the carotid intima-media thickness in subjects with OH and ages <65 years, the researchers also found a mean of <0.9 mm.33 Selcan Koç et al. evaluated the carotid intima-media thickness in a population of 20 to 90 year-olds. Only in the group >70 years was thickness >0.85 mm. It is proposed that the cohort value for the thickening of the intima-media should be evaluated according to age.34 The reproducibility in the carotid intima-media measurements has been evaluated in other studies. High intraobserver and interobserver intraclass correlation coefficients have been found of 0.97 and 0.98, respectively.18,35