Mam why my period is not regular..sometimes 3...

Dear Customer
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What is the possible causes of your medical condition ?
as you mentioned in your question , you have toxic thyorid which usually means you have hyperthyroidsm , and that can be the main cause of your missed period , as it makes your period in the following order :
1. short bleeding time 
2. light period flow 
3. longer period between periods 
What are the other possible causes ?
1. malnutrition and low weight 
2. Hormonal changes 
3. Some medications such as Medroxyprogesterone acetate
4. stress
5. Obesity
6. ovarian cysts 
What to do next ?
1. visit a GYN doctor to do the needed medical examnations and determine the treatment pathway
2. avoid stress and anxeity 
3. healthy life style including healthy regular food and exercise 3- 4 times a week
4. loss weight if you are obese
5. pay attention to your hyperthyroidsm medication and stick to doctor insturction
6. later if your thyroid function is within normal range , and you still have the same issue , doctor may precribe medications such as : primolut nor 
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Kind Regards