Your thyroid gland controls the metabolism of all your cells. Your pituitary gland releases a hormone known as TSH (thyroid stimulating hormone) to stimulate your thyroid. Your thyroid then releases two hormones, T3 and T4, which control your metabolism.
In hypothyroidism (also known as underactive thyroid), your thyroid does not produce enough of these hormones.
There are three types of hypothyroidism: primary, secondary, and tertiary. In secondary hypothyroidism, your pituitary gland isn’t stimulating your thyroid to produce enough hormones. In other words, the problem isn’t with your thyroid. The same is true with tertiary hypothyroidism—the hypothalamus does not produce enough TRH, or thyrotropin-releasing hormone. TRH plays an important role in stimulating the pituitary gland to produce TSH.
In primary hypothyroidism, your thyroid is being stimulated properly. However, it isn’t able to produce enough thyroid hormones for your body to function properly. This means that your thyroid itself is the source of the problem.
The most common cause of primary hypothyroidism is Hashimoto’s disease. This is an autoimmune disease that causes your immune system to mistakenly attack your thyroid.
You might also develop primary hypothyroidism for a number of other reasons.
If you had hyperthyroidism, your treatment may have left you with hypothyroidism. Treatment with radioactive iodine, a common treatment for hyperthyroidism, destroys the thyroid. A less common treatment for hyperthyroidism involves the surgical removal of part of or the entire thyroid. Both result in hypothyroidism.
If you had thyroid cancer, your doctor would have surgically removed your thyroid or part of it to treat the cancer.
Other possible causes of hypothyroidism include:
- insufficient dietary iodine (rare in the United States)
- a congenital disease
- some drugs
- viral thyroiditis
In some cases, a woman might develop hypothyroidism after giving birth. According to the National Institutes of Health, the disease is most common in women and people over 50 years old (NIH, 2012).
The symptoms of hypothyroidism vary widely from person to person. Symptoms typically develop slowly rather than appearing abruptly. They are dependent on the severity of the disease.
At first, you may notice general symptoms including:
- sensitivity to cold
- muscle weakness
Because the thyroid hormones control the metabolism of all your cells, you might also gain weight.
Other possible symptoms include:
- pain in your joints or muscles
- brittle hair and/or nails
- voice hoarseness
- puffiness in your face
As the disease progresses, these symptoms gradually become more severe.
If your hypothyroidism is extremely severe, you might fall into a coma, known as a myxedema coma. This is a life-threatening condition.
If you show physical symptoms of hypothyroidism, your doctor might decide to do tests to check whether you have this condition.
Your doctor will generally use a blood test to check your T4 and TSH levels. If your thyroid is malfunctioning, your pituitary gland will produce more TSH in an attempt to get your thyroid to produce more T3 and T4. An elevated TSH level can indicate to your doctor that you have a thyroid problem.
Treatment for hypothyroidism involves taking medication to replace the missing thyroid hormones. Your doctor will typically start you on a low dose and increase it gradually. The goal is for you to feel good and for your levels of thyroid hormones to return to within the normal range. The state of having a normal thyroid gland function is known as “euthyroid.”
You will continue to take your thyroid medication throughout your life. Your medication replaces the thyroid hormones that your thyroid is unable to produce. It does not correct your thyroid disease. This means that if you stop taking it, your symptoms will return.
Some medications and foods can interfere with your medications. Make sure to tell your doctor about all medications that you take, even over-the-counter ones. Some vitamins and supplements, especially those for iron and calcium, can also interfere with your treatment and should be discussed with your doctor. You may need to cut back on eating anything made from soy and some high-fiber foods.