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Thyroid hormones, triiodothyronine (T3) and thyroxine (T4), have many physiological actions and are essential for normal behavioral, intellectual and neurological development. Although not much work has been published on the effect of stimulants on thyroid hormone secretion, a stimulant medication has been shown to cause a statistically significant drop in thyroxine (T4) in children taking these drugs. Similarly, repeated amphetamine exposure has been shown to cause a decrease in rat plasma T4 levels. Moreover, it appears that dopamine infusion decreases the response of thyroid stimulating hormone (TSH) to thyroid releasing hormone (TRH). Therefore, it would appear that the increase in dopamine caused by stimulant usage could very well result in an attenuation of thyroid hormone release and thereby a stunting of growth. A 7 year-old mild mentally retarded girl taking special education for five years presented with hyperactivity, attentional problems and academic failure to our outpatient clinic. She was diagnosed Attention Deficit /Hyperactivity Disorder (ADHD) according to The Diagnostic and Statistical Manual of Mental Disorders (DSM) V. Then we prescribed short acting methylphenidate 5 mg twice daily. After fifteen days at next medical appointment, she was clinically improved and no side effect was reported. At two months after commencement of this medication during routine laboratory evaluation done by pediatricians, elevated TSH level (7.218 mIU/ml, upper limit of normal TSH assay in the laboratory is 5.50 mIU/ml) was detected. Afterwards, she was consulted to the pediatric endocrinology department. They made further investigations including thyroid ultrasonography, Anti- thyroglobulin antibody test, thyroid peroxidase test and other hormone levels affecting the thyroid gland physiology in order to identify the etiology of elevated TSH level and results of observations revealed that elevated TSH level stemming from stimulant medication. Then methylphenidate treatment was stopped and after two months TSH level became normal (4,64 mIU/ml). This case is considerable because it is important to check thyroid functions in ADHD children on methylphenidate treatment due to its effect on the thyroid gland physiology and importance of thyroid hormones in developing brain.