Thanks for the query.
You once consult gynecologist and go for blood test
By this the possibility of the pregnancy can be found out and also in the absence of pregnancy the cause for delayed periods can be identified.
Thyroid levels can be altered depending on many factors like activity of individual but there may not be seasonal variation.
So you once get examined for other factors also like PCOD, prolactin
After the workup the possible cause can be identified and treated.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429