ABSTRACT Background and Objectives: Vitamin D is an essential secosteroid that plays a crucial role in the homeostasis of a few mineral elements, particularly calcium. Since vitamin D deficiency and thyroid diseases are two important global health problems, we aimed to investigate a possible relationship of vitamin D and calcium levels with hypothyroidism in an Iranian population. Methods: This case-control study was conducted on 175 subjects with hypothyroidism (75 males and 100 females) and 175 euthyroid controls (85 males and 90 females) who were referred to a laboratory in Gorgan, Iran. Serum levels of 25- hydroxyvitamin D, calcium, thyroid-stimulating hormone (TSH), free triiodothyronine (free T3) and thyroxine (total T4) were measured in all participants. Results: Vitamin D and calcium were significantly lower in patients with hypothyroidism (P<0.0001). Free T3 and calcium levels differed significantly among hypothyroid patients based on their vitamin D status (P<0.0001), but vitamin D levels were within sufficient range in all groups. Moreover, there was a positive correlation between free T3 with vitamin D (r= 0.337, P<0.0001) and calcium (r= 0.361, P<0.0001) levels. Conclusions: Our results suggest that there may be a relationship between decreased vitamin D levels and thyroid function parameters. Keywords: Vitamin D Deficiency, Hypocalcemia, Hypothyroidism, Thyrotropin, Thyroxine. Mojtaba Zare Ebrahimabad (MSc) Department of Biochemistry and Biophysics, Metabolic Disorders Research Center, Faculty of Medicine, Golestan University of Medical Sciences, Gorgan, Iran Hanieh Teymoori (MSc) Department of Biochemistry and Biophysics, Metabolic Disorders Research Center, Faculty of Medicine, Golestan University of Medical Sciences, Gorgan, Iran Hamid Reza Joshaghani (PhD) Professor of Clinical Biochemistry, Laboratory Sciences Research Center (LSRC), Golestan University of Medical Sciences, Gorgan, Iran Corresponding author: Hamid Reza Joshaghani Tel: +98-1732436108 E-mail: joshaghani@goums.ac.ir Address: Laboratory Sciences Research Center (LSRC), Golestan University of Medical Sciences, Gorgan, Iran Vitamin D Status and its Relationship with Thyroid Function Parameters in Patients with Hypothyroidism Medical Laboratory Journal, Sep-Oct, 2019; Vol 13: No 5 Original Article This paper should be cited as: Zare Ebrahimabad M, Joshaghani HR [Vitamin D Status and its Relationship with Thyroid Function Parameters in Patients with Hypothyroidism]. mljgoums. 2019; 13(5): 8-12 Received: 30 Jun 2018 Revised: 27 Aug 2019 Accepted: 05 Oct 2019 This work is licensed under a Creative Commons Attribution 4.0 License.