There are actually a few things that can cause thyroid problems Lets look at a few.

First off you can be either hypothyroid (not producing enough t3 and t4 hormones) and hyperthyroid (producing to much t3 and t4)

The most common cause of hypothyroidism is called hasimotos disease and it is an autoimmune condition ( a condition where the body's immune system is attacking the thyroid gland) that leads to deceased thyroid hormone production.

Lack of iodine in the diet can also cause hypothyroidism as iodine is crucial in the production of t3 and t4. Iodine can be found in ionized salt.

Hypothyroidism cause also result from a pituitary problem. The pituitary gland in the brain keeps measurements of t3 and t4 in the blood when the levels are low it secretes a hormone called TSH (thyroid stimulating hormone) this hormone stimulates the thyroid gland to produce and secrete t3-t4.

Hyperthyroidism can also be caused by a pituitary problem a tumor in the pituitary gland causes it to over produce TSH leading to higher than normal levels of t3-t4 in the blood.

Hypothyroidism can also be caused by a decrease in the sensitivity to the hormone TSH the thyroid gland has receptors that detect TSH in the blood and this turns on t3 and t4 production. If there is a loss in the number of these receptors or a problem with them where they are not functioning at their best they may "miss" the signal from the brain to produces more thyroid hormones.

Some simple blood test can help you determine what the case is for you.
If you have low t3 and t4 but high TSH levels then their maybe a loss of hormone receptors or a lack of iodine to produce these hormones.

If TSH is low along with t3 - t4 then the pituitary gland might not be producing enough TSH.

There are several things that can cause a thyroid problem. I will list some of the things.

1. Recently, people were concerned over the nuclear accident due to the earthquakes in Japan.
That is because one of the causes of thyroid problems comes for exposure to radiation.

2. There are certain drugs that will cause thyroid problems. A few of those are lithium or cordarone. These can cause a lack of thyroid hormone, or hypothyroidism. Sometimes the thyroid will produce too much thyroid hormone, or hyperthyroidism.

3. A lack of iodine in the diet has caused thyroid disease in some people. Most people who eat a balanced diet or a multi-vitamin will not have the problem. There is some concern that there is a lack of iodine in soil these days. On the opposite end, getting too much iodine can also cause problems. Some soy products contain too much of it. Some people get carried away and take to many products or supplements that contain iodine such as kelp, etc.

4. Because of other health concerns and cancer, some people will need to have radiation treatments. If these treatments are to the neck, head or chest, it can cause thyroid issues. The tiny little butterfly shaped gland is very sensitive.

5. For some reason, being female increases the chances for thyroid disease, especially if a woman is going through menopause. Hormones get messed up during this time. People over the age of 60 are also at risk for thyroid problems.

6. Probably the most significant cause of thyroid disorders is genetic. Those who have the problem will usually find that there are other family members who also suffer from the same thing. For example, I inherited a goiter on my thyroid from my grandfather and so did two of my cousins. They have all had their thyroids removed, and my doctor is trying to help me through thyroid hormone replacement so that I can avoid it. Unfortunately, I have two children who have thyroid disease.

7. Some people will have pituitary gland problems or endocrine disease. The thyroid will usually be affected by this. There are those who will get a disorder called Graves' disease which causes hyperthyroidism. The thyroid will be treated with radioactive iodine. This will leave the person with a hypothyroid condition after these treatments. Having a hypothyroid condition is easy to treat with thyroid hormone replacement.

8. Smoking will sometimes cause thyroid disease.

9. Thyroid disorder is considered an autoimmune disease. Usually, if a person has an autoimmune disease, they have an increased risk of developing another one. Those who have been diagnosed with Fibromyalgia or Chronic Fatigue Syndrome will often have thyroid disease.

I hope this helps.