Overactive thyroid makes you feel constantly hot, moody and lose weight whilst eating shed loads.
Underactive makes you feel cold all the time, memory loss, weight gain.
My Missus has/had an overactive thyroid. The docs ended up zapping it somehow and she now relies on tablets to regulate hormones.
If it's overactive then you'll find that you have tonnes of energy and you're awake all of the time, almost a "burnout" type scenario. You'll probably also find that you have a very high metabolism - ie, you shit a lot. Despite popular belief, and overactive thyroid actually makes you LOSE weight.
Here's a bunch of crap from the Bupa site. If you reckon you've got problems, go see the doc. Don't ask us fuckwits!
Symptoms
Hyperthyroidism can cause many different symptoms and the overall pattern can vary for different people, depending on their age and the amount of excess hormone being released. In general, the symptoms relate to a "speeding up" of the body's metabolism and they include the following:
feeling nervous, irritable or emotional
tremor (shaking)
sleeping poorly
being intolerant of heat and sweating more than usual
losing weight despite increased appetite (10% of people gain weight)
feeling tired and having weak muscles
increased heart rate or palpitations
having infrequent periods or problems with fertility
having more frequent bowl movements or diarrhoea
shortness of breath, especially when exercising
loss of hair or finer hair than usual
swelling of the thyroid gland in the neck - a goitre
http://hcd2.bupa.co.uk/fact_sheets/html/overactive_thyroid.html