Thyroid hormones (THs) have different effects on the heart and peripheral vascular system. Several changes occur across different status of thyroid dysfunctions. Overt hyperthyroidism (OHyper) induces significant changes in cardiac functions. Untreated OHyper is a cause of heart failure (HF). B-type Natriuretic Peptide (BNP) synthesized primarily in the myocardium. Thus, it is recommended as a diagnostic and prognostic marker for patients with HF. The present study was conducted to measure the effects of OHyper on the BNP level. A case-control study was conducted at a private clinic; Mosul Daily Surgical Clinic, Mosul, Nineveh, Iraq. From 1st Oct., 2020 to 1st Mar., 2021. Thirty (30) apparently healthy subjects with normal thyroid functions; Euthyroid (EU) subjects regarded as control group. Another newly diagnosed thirty (30) OHyper patients were enrolled in this study after exclusion of any facts that affect BNP level as cardiac disease, hypertension, patient taking angiotensin converting enzyme inhibitor, and pregnant ladies. They were diagnosed on the basis of thyroid function tests (TFTs); Thyroid Stimulating Hormone (TSH), Free Triiodothyronine (FT3) and Free Thyroxine (FT4). Plasma level of BNP were measured in OHyper and EU subjects’ group. Plasma BNP level is higher in patients with OHyper than EU individuals with mean values of 27.40± 35.59 and 21.68± 28.57 pg ml-1, (p>0.05) respectively. Moreover, there was a positive correlation between plasma BNP and serum FT4 level in EU (r=0.31, p=0.01) and in OHyper patients (r=0.57, P=0.001). OHyper affect plasma BNP level possibly influencing the secretion of this peptide. Therefore, thyroid functions have to be considered when evaluating any high plasma BNP level. Manuscript profile