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Samia A. Eleiwe*, Nibras Hatim Khamees** and Shilan Hussein Karim**

*  Department of Anatomy; AlKindi College of Medicine; University of Baghdad.
** Department of Anatomy, School of Medicine, Faculty of Medical Sciences, University of Sulaimani.

Submitted: 1/3/2013; Accepted: 17/9/2013; Published 1/6/2014



The thyroid glands function and structure are influenced by several psychomotor stimulants like: temperature, drugs, chemicals and light. The purpose of this study was to investigate the effect of continuous darkness on the thyroid glands of male rats. Despite so many previous works to explore the effect of exogenous or endogenous melatonin on thyroid, there are few studies dealing with the outcome of progressively increasing phases of darkness on thyroid architecture, according to the available literatures.


This work is intended to study the effect of rising period of continuous darkness on thyroid tissues in adult male rats.


Adult Wister albino rats were kept in total 24 hours darkness for successive 4 phases. These rats were divided into 8 groups. Group II, III, IV and V were cited in continuous darkness for 2, 4, 6 and 8 weeks in that order. Group Iɑ, Group Ib, Group Ic , and Group Id were control groups of group II, III, IV and V likewise. After the last day of the dark phase dedicated for each group, the animals were sacrificed under effect of anesthesia. The thyroid gland was separated, weighed and right lobe was practiced to study its structural changes.


The results illustrated no key structural effect by short and medium phases of darkness, while on long phases; there was detrimental effect on thyroid tissues.


The continuous darkness for a long time has undesirable histological and anatomical changes on the thyroid tissues of the adult male rats in a manner correlated with the length of publicity.


Thyroid, Melatonin, Darkness, Endocrine.


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