Can My Underactive Thyroid Become Overactive?

According to the American Thyroid Association, thyroid disease affects nearly 20 million¹ people in the United States. While almost 12% of the US population will develop a thyroid condition during their lifetime, 60% of people with thyroid disease are unaware of their condition.

This increases the risk of developing severe medical conditions, such as cardiovascular diseases, osteoporosis, and infertility.

Some people experience underactive thyroid, or hypothyroidism, whereas others develop overactive or hyperthyroidism.

It’s important to know the difference between these conditions and recognize any symptoms that indicate you need to see a doctor to evaluate the health of your thyroid.

What's the difference between hypothyroidism and hyperthyroidism?

Your thyroid gland is a small gland located in the front of your neck. It produces hormones that regulate your body's metabolic rate. The hormones produced by your thyroid gland have an enormous impact on your health, affecting your heart, muscles, digestive system, and nearly every other bodily function.

When your thyroid gland produces more hormones than your body needs, you have an overactive thyroid or hyperthyroidism. In contrast, when your thyroid gland doesn't make enough hormones, you have hypothyroidism, or an underactive thyroid.

The two diseases typically present different signs and symptoms. Some symptoms may overlap, such as enlargement of the thyroid gland, called a goiter.

When you have hyperthyroidism, and your thyroid gland produces too many hormones, your body cells work faster than usual. The increased activity in your cells and organs can cause your heart rate to increase and affect your intestines, causing more frequent bowel movements or diarrhea.

Some of the more common symptoms experienced by people with hyperthyroidism may include:

On the other hand, hypothyroidism doesn't usually cause any noticeable symptoms in the early stages. However, as the condition progresses, an underactive thyroid gland can lead to several health problems, including obesity, joint pain, infertility, and heart disease. 

The lack of hormones slows your metabolism, affecting your cells and organs. This may show up as a slower than usual heart rate, or your intestines may begin to work sluggishly, causing constipation.

Some of the common symptoms associated with hypothyroidism include:

  • Weight gain

  • Depression

  • Fatigue

  • Joint and muscle pain

  • Slowed heart rate

  • Trouble tolerating cold

  • Dry skin or dry, thinning hair

  • Heavy or irregular menstrual periods or fertility problems

Can you develop hyperthyroidism if you were previously diagnosed with an underactive thyroid?

Cases of spontaneous alternating from hyperthyroidism to hypothyroidism are rare, with most occurring several years after the person stops their antithyroid medications.

In addition, people with overactive thyroids often undergo a total thyroidectomy or radioactive iodine treatment. These treatments eliminate the possibility of the thyroid function changing spontaneously.

The most common reason a person with hypothyroidism may develop hyperthyroidism is too much thyroid hormone medicine. Though some people may intentionally take too much thyroid hormone, the change in hormone levels results more commonly from the prescribed dosage of hormone medicine accidentally being too high.

Known as medication-induced hyperthyroidism, this often happens when the dose is not adjusted correctly based on follow-up blood test results.

If you have hypothyroidism and develop persistent hyperthyroidism symptoms, your doctor should test for both conditions. Otherwise, the condition may be missed and explained as an overreplacement of thyroid hormones.

Treatment options

Treating hypothyroidism involves replacing the hormones that your thyroid gland can no longer produce. Your doctor will usually prescribe levothyroxine, a thyroid hormone medication identical to a hormone produced by a healthy thyroid gland. The medicine is available in liquid, pill form, or soft gel capsule. It’s typically taken in the morning before breakfast.

Your doctor will follow up with blood tests every six to eight weeks after you begin the treatment. They will then adjust your dosage as necessary, and each time it is changed, your doctor will order another blood test. 

When you have found the dose that gives you the desired results, they will perform another blood test in six months and then yearly after that. Hypothyroidism can usually be controlled entirely with thyroid hormone medicine if you take it as prescribed.

Treating hyperthyroidism involves bringing the production of thyroid hormone levels back to normal. This is necessary to prevent long-term health problems and relieve uncomfortable symptoms. The usual treatment for hyperthyroidism is medicine, radioiodine therapy, or thyroid surgery.

Unfortunately, there is no single treatment that works for every type of hyperthyroidism, and your treatment will depend on the cause and severity of your disease.

When deciding upon the appropriate treatment, your doctor will consider potential side effects, your age, allergies, and other medical conditions, such as pregnancy or heart disease. 

When to see your doctor

Thyroid disease often goes unnoticed in the early stages. Left untreated, it can lead to severe medical complications. A common sign of a thyroid issue is a lump or swelling in the neck near the thyroid gland. 

Early warning signs of a thyroid problem may include:

  • Unexpected weight gain or loss

  • A rapid or slow heart rate

  • Sensitivity to heat or cold

  • Fatigue

If you experience any of these symptoms, consult your doctor. They will perform a physical examination, ask about your symptoms, and request blood tests to make a diagnosis. If they suspect thyroid disease, they may refer you to a doctor specializing in thyroid conditions. If you are receiving treatment and your symptoms change, contact your doctor immediately for a review.

The lowdown

Thyroid disease affects a significant portion of the US population. Because thyroid disease usually presents few symptoms in the early stages, it often goes undiagnosed, increasing your risk of developing a severe medical condition.

Sometimes, if you have an underactive thyroid that becomes overactive, the cause is too much thyroid hormone replacement therapy.

While it's rare for someone with an underactive thyroid to develop spontaneous hyperthyroidism without excess thyroid hormone medication, it is possible.



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