Endocrine Abstracts

Background: Subclinical hypothyroidism (SH) is a frequent clinical condition with a prevalence of 3–15% in general population. It is defined by elevated TSH levels with normal thyroid hormones [free thyroxine (FT4) and free triiodothyronine (FT3)]. Similar to prolactin, high TSH levels may be caused by macroTSH, a large molecular sized TSH with a low bioactivity. The aim of the study was to assess the prevalence of macroTSH in patients with subclinical hypothyroidism.

Subjects and methods: Blood samples were obtained from 500 adult patients with subclinical hypothyroidism (TSH>5 mUI/ml and FT4 within the reference interval: 8–17 ng/l) between September 2017 and September 2018. The presence of macroTSH was assessed by precipitating 250 μl of serum treated with 250 μl of 25% polyethylene glycol (PEG). The precipitable TSH (%) was calculated using the formula: [1-(2* post-PEG TSH/pre-PEG TSH)*100]. Samples with a precipitable TSH >75% were considered as macroTSH positives.

Results: Of 500 patients (mean age 53±18 years), 366 (73%) were females. The median of pre-PEG TSH was 6.5 mUI/ml (IQR: 5.5-8.5) with a mean FT4 of 11.6±1.85 ng/l. The median post-PEG TSH was 3.1 mUI/ml (IQR: 2.4-4.2) with a mean precipitable TSH of 53±12%. Three patients (0.6%) had macroTSH and 26 (5.2%) a borderline precipitable TSH between 70 and 74%. MacroTSH positive patients were all young females, aged between 31 and 41 years with negative antithyroid antibodies [antithyroglobulin (AbTg) and antithyroidperoxidase (AbTPO)] and not receiving any related thyroid therapy. Pre-PEG TSH levels ranged between 5.7 and 12.9 mUI/ml, whereas the post-PEG TSH ranged between 0.98 and 3 mUI/ml. Regarding 26 patients with borderline precipitable TSH, 20 (77%) were females with a mean age of 58±16.3 years. Evaluation of AbTg and AbTPO was available in 13 patients: 12 out of them (92%) had at least one positive antibody and 11/13 was receiving thyroid substitutive therapy. The median pre-PEG TSH was 6.2 mUI/ml g(IQR: 5.4-7.9), while the median post-PEG TSH was 1.77 mUI/ml (IQR: 1.5-2.3).

Conclusion and discussion: The prevalence of macroTSH in our cohort of patients with SH was 0.6%. This result is in line with other previous studies that reported a prevalence between 0.6 and 1.62%. Despite the low prevalence, the assessment of macroTSH could be useful in patients with subclinical hypothyroidism and negative antithyroid antibodies in order to better evaluate the need of a chronic substitutive therapy.