Certain epidemiological studies revealed correlation between hardwater consumption (with high calcium) and thyroid size of the population, though the possible alterations in thyroid physiology upon calcium exposure are still inconclusive. Adult maleWistar strain rats were subjected to calcium treatment at the doses of 0.5 g%, 1.0g% and 1.5g% calcium chloride (CaCl2) for 60 days. The parameters studied were - thyroid gland weight, histopathology, histomorphometry; thyroid peroxidase (TPO), 5'-deiodinase I (DI), sodium-potassium adenosine triphosphatase (Na +-K+-ATPase) activities; serum total and free thyroxine (tT4, fT4), total and free triiodothyronine (tT3, fT3), thyroid stimulating hormone (TSH) levels. Enlargement of thyroid with hypertrophic and hyperplastic changes, retarded TPO and 5'-DI but enhanced Na+-K+-ATPase activities, augmented serum total and free T4 and TSH but decreased total and free T3 levels and low T3/T4 ratio (T3:T4) were observed in the treated groups. All these findings indicate development of goitrogenesis upon exposure to excessive dietary calcium. © 2012 Elsevier B.V. All rights reserved.