Objectives To investigate the thickness of the shoulder tendons and the measurement of acromiohumeral distance (AHD) in patients with Hashimoto's disease (HD). Material and Methods Twenty-eight patients with subclinical hypothyroid HD, 40 patients with euthyroid HD, and 51 healthy subjects were included. The thicknesses of biceps brachii, subscapularis, supraspinatus, infraspinatus tendons at both shoulders were evaluated with ultrasonography. Serum levels of thyroid stimulated hormone (TSH), free tri-iodothyronine, free thyroxine (FT4), anti-thyroid peroxidase (TPO) and anti-thyroglobulin (anti-TG) antibodies levels were measured. Results Height, weight, body mass index (BMI), free T3 and free T4 levels were similar between the three groups (P = .839,P = .205,P = .374,P = .430 andP = .497, respectively). Biceps brachii, supraspinatus and infraspinatus tendon thicknesses in dominant arm and biceps brachii, subscapularis and infraspinatus tendon thicknesses in non-dominant arm were significantly increased in euthyroid HD compared to healthy controls (P = .003,P = .030,P P = .035,P = .042,P < .001, respectively). Biceps brachii tendon thickness in dominant arm and subscapularis and supraspinatus tendon thicknesses in non-dominant arms were significantly increased in subclinical hypothyroid HD compared to healthy controls (P = .025;P = .046,P = .017, respectively). However there was no such difference between euthyroid HD and subclinical hypothyroid HD groups (P < .05). There was low correlation between biceps brachii tendon thickness and free T4 level in non-dominant shoulder in patients with HD (r = .272P = .030). For the rest of the tendons, there was no correlation between TSH, anti-TPO, anti-TG levels and tendon thicknesses in patients with HD. Conclusions This study suggests that thyroid autoimmunity in HD may lead to an increase in thickness of shoulder tendons.