One of the causes of an underactive thyroid can be a lack of iodine, so it is easy to think that increasing your iodine intake might help (e.g. by eating more oily fish). Without proper medical advice, however, this could be a risky strategy because not all cases of hypothyroidism are due to a lack of iodine and an excess of iodine could lead to more health problems.
A trip to the doctor (or, realistically, several trips to the doctor) will result in a prescription for Thyroxin tablets. It might take some time to get the dosage right but this should deal with the problem.
Link to BUPA information:
http://hcd2.bupa.co.uk/fact_sheets/html/underactive_thyr oid.html
Hoping this helps,
Chris