Subjects

This study initially enrolled 236 outpatients and was conducted from January to December 2013 at the Department of Endocrinology in Beijing Chaoyang Hospital. HO is a thyroid hormone deficiency and can develop as a primary disease of the thyroid gland itself. An elevated thyrotropin (TSH) level, usually above 10 μIU/ml, along with a subnormal free thyroxine (FT4) level, characterizes overt HO [23]. SHO is defined as a serum TSH concentration above the statistically defined upper limit of the reference range and when serum FT4 and free triiodothyronine (FT3) concentrations are within the normal reference ranges [24, 25]. This designation is only applicable when thyroid function has been stable for several weeks, the hypothalamic–pituitary–thyroid axis is normal, and there is no recent or ongoing severe illness. Exclusion criteria were as follows: patients with cardiovascular disease, hypertension, diabetes mellitus or impaired glucose tolerance, renal diseases or other endocrine diseases. Therefore, 72 patients were excluded. The final study cohort consisted of 164 patients among those individuals who were initially enrolled, including 73 and 91 patients with HO and SHO, respectively. None of the patients received any treatment. The control group included 94 normal, non-HO volunteers who were seeking routine medical care at the physical examination centre of Beijing Chaoyang Hospital.

HHcy was defined as a plasma Hcy level greater than 15 μmol/l [17]. Based on the presence of HHcy, each group was subdivided into two subgroups. The normal reference value of Hcy was less than 15 μmol/L. In the HO group, the patients with a plasma level of Hcy > 15 μmol/l were termed the H-HO group (n = 36), and the other patients made up the N-HO group (n = 37). Similarly, we divided the SHO group into the H-SHO (n = 32) and N-SHO (n = 59) groups, and the control group was split into the H-control (n = 18) and N-control (n = 76) groups.