The most common cause of low thyroid levels in the United States is an autoimmune condition called Hashimoto’s disease, where the immune system gradually destroys thyroid tissue. But autoimmunity is only one of several reasons your thyroid might underperform. Nearly 5 out of 100 Americans ages 12 and older have some degree of hypothyroidism, and the causes range from nutritional deficiencies to medications to problems with the pituitary gland. Women are much more likely to develop the condition, and risk increases after age 60.
Hashimoto’s Disease
In Hashimoto’s disease, the immune system produces rogue antibodies that target a protein called thyroid peroxidase (TPO), which plays a key role in making thyroid hormones. Over time, this immune attack damages enough thyroid cells that the gland can no longer keep up with the body’s demand for hormones. Most people with Hashimoto’s have detectable TPO antibodies in their blood, which is how doctors typically confirm the diagnosis.
The destruction happens slowly, often over years. You might have normal thyroid levels for a long time before the damage accumulates enough to cause symptoms like fatigue, weight gain, or feeling cold. Because the onset is so gradual, many people don’t realize anything is wrong until routine bloodwork catches an elevated TSH level.
Iodine Deficiency
Your thyroid needs iodine to build its hormones. The gland pulls iodine from the bloodstream and incorporates it into the hormones that regulate your metabolism, energy, and body temperature. When iodine intake falls short, the thyroid simply can’t produce enough hormone. In response, the pituitary gland cranks out more TSH to try to compensate, and that persistent stimulation can cause the thyroid to enlarge into what’s known as a goiter.
Iodine deficiency is uncommon in the U.S. thanks to iodized salt, but it remains a significant cause of hypothyroidism worldwide. Adults need about 150 micrograms of iodine daily. Pregnant women need considerably more (220 micrograms), and breastfeeding women need 290 micrograms, because the developing baby depends entirely on the mother’s iodine supply.
Thyroid Surgery and Radioactive Iodine Treatment
If you’ve had part or all of your thyroid removed surgically, the remaining tissue may not produce enough hormone on its own. Total thyroidectomy guarantees permanent hypothyroidism, while partial removal carries a variable risk depending on how much gland is left.
Radioactive iodine therapy, commonly used to treat an overactive thyroid or thyroid cancer, works by destroying thyroid cells from the inside. About 31% of patients treated with radioactive iodine for overactive thyroid nodules develop hypothyroidism afterward. This can happen within months or emerge years later, which is why long-term monitoring is standard after treatment.
Medications That Affect the Thyroid
Several widely prescribed drugs can suppress thyroid function as a side effect. Lithium, used for bipolar disorder, causes hypothyroidism in up to 20% of patients by blocking the thyroid’s ability to release its hormones. Amiodarone, a heart rhythm medication, is another well-known culprit. A single 200-milligram tablet delivers 45 times the recommended daily intake of iodine, and that iodine overload paradoxically shuts down hormone production in some people. Between 14% and 18% of patients on amiodarone develop thyroid problems, with hypothyroidism typically appearing within six to twelve months of starting the drug.
Newer cancer immunotherapy drugs called checkpoint inhibitors carry an even higher risk. Up to 40% of patients treated with certain combinations develop thyroid dysfunction. The typical pattern starts with a brief phase of excess thyroid hormone, followed by a drop into hypothyroidism that is often permanent.
Pituitary Gland Problems
Your thyroid doesn’t operate independently. It takes orders from the pituitary gland, a pea-sized structure at the base of the brain that releases TSH to tell the thyroid how much hormone to make. When the pituitary is damaged, it may stop sending that signal, and the thyroid idles even though the gland itself is perfectly healthy. This is called central hypothyroidism.
Pituitary tumors, especially noncancerous growths called adenomas, are the most common cause. These can compress the hormone-producing cells, cut off blood flow between the hypothalamus and pituitary, or in rare cases bleed suddenly in an event called pituitary apoplexy. Other causes include cysts, head trauma, radiation therapy to the brain, and certain infiltrative diseases. Central hypothyroidism is trickier to diagnose because TSH levels may look normal or only slightly off, unlike the clearly elevated TSH seen in primary thyroid failure.
Postpartum Thyroiditis
Some women develop thyroid inflammation in the months after giving birth. The hypothyroid phase typically hits between four and eight months postpartum and can last nine to twelve months. It’s easy to mistake the fatigue, brain fog, and mood changes for normal postpartum adjustment, which means many cases go undiagnosed.
The good news is that most women recover normal thyroid function within 12 to 18 months. However, about 20% of women who enter the hypothyroid phase remain permanently hypothyroid and need ongoing treatment. Women who’ve had postpartum thyroiditis with one pregnancy are at higher risk of it recurring with future pregnancies.
Congenital Hypothyroidism
Over a thousand babies are born each year in the United States with a thyroid that doesn’t work properly from birth. In 80% to 85% of cases, the thyroid gland simply didn’t develop correctly during pregnancy. The remaining 15% to 20% of cases are caused by genetic changes that prevent the thyroid from making enough hormone. Babies with Down syndrome have a higher-than-average risk. Newborn screening catches the condition early, which is critical because untreated congenital hypothyroidism can cause serious developmental delays.
How Low Thyroid Levels Are Detected
The standard screening test measures TSH in your blood. A normal TSH falls roughly between 0.45 and 4.12 mIU/L. When your thyroid isn’t making enough hormone, the pituitary pumps out more TSH to compensate, so a high TSH is the first signal that something is off.
There’s an important distinction between full-blown hypothyroidism and a milder version called subclinical hypothyroidism. In the subclinical form, TSH runs between about 5 and 10 mIU/L, but your actual thyroid hormone levels remain in the normal range. You may have no symptoms at all, or you may have vague ones like mild fatigue. Not everyone with subclinical hypothyroidism needs treatment. Doctors are more likely to recommend medication if your TSH reaches 10 or higher, if you’re younger and symptomatic, or if you’re trying to conceive. For women undergoing fertility treatments, guidelines recommend bringing TSH down to 2.5 mIU/L or below.
When TSH climbs above 10 mIU/L, the risk of heart disease and heart failure increases, and treatment is generally recommended regardless of symptoms. Your doctor will usually repeat the blood test within three months before starting any medication, since TSH levels can fluctuate temporarily due to illness, stress, or other factors.