Hi, Thanks for asking. T4 is a hormone that is normally produced by your thyroid gland to regulate the body s energy and metabolism in response to stimulation by TSH (Thyroid Stimulating Hormone) secreted by Pituitary gland. When the thyroid gland is not secreting T4 (Lower t4)adequately to the body s needs (Hypothyroid State), body s energy and metabolism are affected (That explains the fatigue). In response to this, the Pituitary gland secretes TSH (Thyroid Stimulating Hormone) for the thyroid to secrete increased T4.(So, the High TSH) Euthyrox (Levothyroxine) is a replacement for a T4 is a hormone, given when the thyroid does not produce enough of this hormone on its own. The less serious side effects of Euthyrox are •Abdominal Pain with Cramps •Alopecia(Hair loss) •Anxiety •Diarrhea •Fatigue •Fever •Flushing •Headache Disorder •Hyperhidrosis(Increased sweating) •Increased Appetite •Insomnia •Irregular Menstrual Periods •Irritability •Muscle Weakness •Nervousness •Palpitations &Tremors You need to be given some replacement for the lesser T4 in your body, for sure.Your Physician has started you on a safe, low dosage of Euthyrox only. Probably it will take some time for you to get accustomed for the medication. So If the symptoms persist, we shall have a review but You should do well in a week s time. Take care, Regards, Dr.K.Muthukumar MD