With the deepening degree of population aging and the rapid development of socioeconomic structure, HTN and SCH have gradually become important factors affecting the health of residents. Studies have pointed out that HTN is closely related to SCH prevalence, but the pathogenesis of HTN resulting in SCH is not yet clear.
The prevalence of SCH in HTN population under 60 years old in Gansu Province was 28.95%, significantly higher than that in normal population (17.58%). However, the prevalence rate of SCH in the whole age group of HTN population was always significantly higher than that in the normal population (24.34% vs 17.28%, χ2 = 11.813;31.95% vs 17.86%, χ2 = 40.039,P < 0.05), regardless of whether the TSH level was considered to increase with age physiologically (the SCH diagnostic criteria for people over 65 years old consider TSH > 8.86 mIU/L alone). The diagnostic criteria for SCH in the elderly are still controversial. Professor Biondi et al. have shown that serum TSH levels in elderly patients may exceed the upper limit of the traditional reference range of 4–5 mU / L, which may lead to an overestimation of the true prevalence of subclinical hypothyroidism in people over 70 years of age.9 Consistent with our study, we found that the prevalence of SCH was higher in people over 65 years of age without considering the effect of age on TSH(normal: 28.00%、HTN: 34.73%); the prevalence of SCH in people over 65 years of age decreased significantly when considering the physiological increase of TSH caused by age (normal: 4.00%、HTN: 7.19%). Some studies have shown that in the absence of thyroid diseases, the elderly with slightly elevated serum TSH did not increase the incidence of SCH and the risk of death, suggesting that we should update the diagnostic criteria of TSH according to the reference range of TSH in the elderly population in the region.10 In calculating the prevalence of SCH in the elderly, it is necessary to reconsider the boundary value of TSH to avoid the misdiagnosis of SCH in the population, so that the calculation of the prevalence of SCH is more reasonable, which is helpful for the diagnosis and treatment of clinicians. However, in either case, the prevalence of SCH in Gansu HTN population was lower than that in Chongqing (31.70%) and higher than that in India (8.20%).11,12 This may be related to the geographical environment, living habits and many other factors in Gansu province, which should be paid attention to. Meanwhile, we found that the grade 3 HTN population had the highest prevalence among the three subtypes of HTN, and HTN population prevalence were higher than the SCH prevalence in the normal population,regardless of considering the effect of age on TSH.
Further gender-stratified analysis of the HTN population found that the prevalence of SCH was always higher in women than in men, consistent with the findings of He et al, who also observed that HTN women were more likely to develop SCH than men.13 We also found that TPOAb and TgAb levels were higher in SCH group than in normal group, and the prevalence of SCH in thyroid antibody-positive patients was significantly higher than that in thyroid antibody-negative patients in the total HTN population and grade 2 HTN population, while high TPOAb and high TgAb were risk factors for SCH in HTN population. This may be because hypertension by enhancing thyroid inflammation, antiperoxidase antibody cause autoimmune thyroiditis, because thyroid peroxidase has an important role in the synthesis of thyroid hormone, caused by antiperoxidase antibody inhibition, potential thyroid damage caused by antiperoxidase antibody may reduce the effectiveness of thyroid hormone, so that serum TSH concentration compensatory rise, leading to the occurrence of SCH.14 Meanwhile, we found that thyroid-related antibody levels were significantly higher in women than in men in both normal and SCH populations, suggesting that HTN women need more clinical attention.
Studies have found that high FPG, high TG and high LDL-C are the risk factors for SCH in the HTN population, which may be due to the long-term hypertensive state, hypersympathetic activity, causing insulin resistance, leading to abnormal glycemic and lipid metabolism in the body.15 People with abnormal glucose and lipid metabolism are more prone to autoimmune responses and thyroid cell destruction. At the same time, leptin levels are high in patients with abnormal glucose and lipid metabolism, which may affect the hypothalamic-pituitary-thyroid axis pathway through Janus activated kinase (JAK) -2 / signaling and transcriptional activation (STAT), so as to stimulate TSH synthesis and affect thyroid function.9 This finding suggests that HTN patients should be paid attention to thyroid function-related indicators.
Moreover, we found lower heart rate in SCH patients in HTN population. Klein et al also found lower heart rate in SCH patients,16 and the Logistic regression analysis model also showed that heart rate is a protective factor for SCH in HTN population, probably because HTN is a risk factor for atherosclerosis,17 and the diffuse atherosclerosis of coronary artery also often lead to lower coronary blood flow reserve, which may lead to dilated myocardial ischemia and perfusion, heart blood supply oxygen, so the heart rate increased.18 SCH patients often with slower heart rate, cardiac output and systemic vascular resistance increase,4 so elevated heart rate in HTN patients may prevent SCH.
The prevalence of SCH is high in the HTN population, and studies show that HTN is a risk factor for atherosclerosis, which can affect the blood supply of the thyroid gland, and thus inhibit the thyroid function.19,20 Other studies have shown that the vast majority of HTN patients have different forms of metabolic disorders, and the metabolic disorders in the body can affect the secretion of TSH, and then affect the occurrence of SCH. The metabolic disorders in HTN patients can also hinder the energy utilization of thyroid follicle cells, resulting in iodine pump not working normally, and the body TSH level reaction increases.20,21 In addition, HTN is an immune-inflammatory mechanism mediated disease.22 Increased inflammatory cytokines such as interleukins in patients can inhibit the synthesis of thyroid peroxidase mRNA, thereby inhibiting the synthesis of thyroid hormones.23 Under the regulation of the hypothalamus-pituitary-thyroid axis, the body TSH level is increased to maintain the normal circulating thyroid hormone level.24