Everyone feels run-down now and then.
But if your body is running low on thyroid hormone, you may not only feel run-down; you may also feel downright sluggish, slow, cold and tired.
Here's why: Thyroid hormone controls various functions in your body, including how fast your cells burn energy, how fast tissues grow, and how fast electrical impulses travel in your nerves and muscles.
When your thyroid gland doesn't produce enough hormone, these processes slow down. And that's what can make you feel drained and exhausted, among other things.
Several causes
When the thyroid produces too little thyroid hormone, it's called hypothyroidism.
In most cases, the culprit is an autoimmune disease, when a person's immune system mistakes thyroid gland cells for a threat and attacks them. According to the <a href="[BASEDOMAIN]/healthinfo/bouncelink.aspx?shortcut=amthyroid" target="_blank">American Thyroid Association</a> (ATA), the most common forms of autoimmune thyroiditis are Hashimoto's thyroiditis and atrophic thyroiditis.
Other causes of hypothyroidism include:
<ul>
 <li>Surgical removal of all or part of the thyroid gland. This may be necessary to treat conditions such as thyroid nodules, thyroid cancer or Graves' disease. </li>
 <li>Radiation treatment. Radiation may be used to destroy the thyroid gland for people who have Graves' disease, nodular goiter or thyroid cancer. Radiation treatment for Hodgkin's disease, lymphoma or cancer of the head and neck may also damage the thyroid. </li>
 <li>Congenital defects. In some cases, babies are born without a thyroid or with a thyroid that is only partly formed.</li>
 <li>Medications. Drugs such as lithium, amiodarone, interferon alpha and interleukin-2 can keep the thyroid gland from working normally. </li>
</ul>
Sometimes, hypothyroidism occurs after pregnancy or because of a viral infection, but usually these cases are temporary, according to the <a href="[BASEDOMAIN]/healthinfo/bouncelink.aspx?shortcut=endocrin" target="_blank">American Association of Clinical Endocrinology</a>&nbsp;(AACE).
Slow symptoms
A failing thyroid may not cause many symptoms early on. That's because, for a while at least, the body may try to compensate by stimulating the thyroid to produce more hormone (similar to how you might push the accelerator to give a car more gas when driving up a hill). 
But as hormone production decreases and the body's metabolism slows down, many symptoms may develop, including:
<ul>
 <li>Pervasive fatigue. </li>
 <li>Feeling chilly, especially when other people are comfortable. </li>
 <li>Dry, itchy skin. </li>
 <li>Weight gain and fluid retention. </li>
 <li>Hair loss. </li>
 <li>Constipation. </li>
 <li>Dry, brittle hair and nails. </li>
 <li>Slow heart rate. </li>
 <li>Puffy face. </li>
 <li>Sore muscles. </li>
 <li>Depression. </li>
 <li>Heavy and/or irregular menstrual periods. </li>
 <li>A goiter (enlargement of the thyroid) that may look or feel like a lump in the neck. </li>
</ul>
Some of these symptoms&mdash;such as feeling tired and overworked&mdash;could be caused by something as simple as stress. But the only way to find out for sure is by checking with your doctor and getting tested.
Back up to speed
Fortunately, treatment for hypothyroidism is fairly straightforward.
The missing thyroid hormone is usually replaced in pill form with a drug called levothyroxine (that's the generic name; several brand names are also available). 
Taken once daily, the medication fully replaces the thyroid hormone and successfully treats symptoms in most people who take it, according to the ATA. If your doctor prescribes the medicine, it's very important to take it consistently&mdash;in the same manner every day. The body is very sensitive to even small changes in thyroid hormone levels, the AACE says.
Since hypothyroidism is usually permanent and often progressive, it must be treated indefinitely. 
Medication doses may have to be adjusted over time, so regular checkups and blood tests are needed to help ensure the proper dose is being given.
Checking in
If you think you have a thyroid problem, let your doctor know. A simple blood test could uncover the cause of your symptoms. 
Reviewed 10/10/2023

An underactive thyroid may need a boost

A guide to the symptoms, causes and treatments of hypothyroidism.