A total of 1000 patients with primary hypothyroidism on stable LT4 doses and in a euthyroid state for ≥ 6 months, out of whom 720 met the inclusion criteria while 280 were excluded (88 patients with hypothyroidism resulting from thyroidectomy, 24 patients with radioiodine ablation, 106 patients with medical conditions or on medications known to interfere with thyroid hormone absorption, and 62 patients with incomplete data). This evidence concerns the gene TG and hypothyroidism.