An underactive thyroid means your thyroid gland isn’t producing enough hormones to keep your body’s metabolism running at its normal pace. The thyroid is a small, butterfly-shaped gland at the base of your neck, and the hormones it releases influence virtually every organ system in your body, from your heart and brain to your muscles and digestive tract. When production drops, everything slows down: you burn less energy, generate less body heat, and processes like digestion and even thinking become sluggish. The medical term is hypothyroidism, and it’s one of the most common hormonal conditions.
How Thyroid Hormones Affect Your Body
Your thyroid produces two main hormones, commonly called T3 and T4. These hormones enter your cells, switch on genes that control your metabolic rate, and determine how much oxygen and energy your body uses at rest. They increase your core body temperature, help your body process carbohydrates and build proteins, and keep your heart pumping at a steady rate. They also support normal brain function, mood regulation, and muscle strength.
When thyroid hormone levels fall too low, the effects ripple across your entire body. Your basal metabolic rate drops, which means you burn fewer calories and produce less heat. That translates into cold intolerance, fatigue, and weight gain. Lower hormone levels also reduce activity in your sympathetic nervous system, the branch responsible for your “fight or flight” response, leading to a slower heart rate, decreased sweating, and sluggish digestion that shows up as constipation. Cholesterol levels often rise because the body can’t clear fats from the blood as efficiently. In the brain, low thyroid hormones can impair memory, slow speech, and cause persistent sleepiness.
Common Causes
In developed countries, the most common cause is an autoimmune condition called Hashimoto’s thyroiditis. In Hashimoto’s, your immune system mistakenly attacks the thyroid’s hormone-producing cells, gradually replacing functional tissue with scar tissue. Over months or years, the gland loses its ability to keep up with your body’s demand for thyroid hormones. The exact trigger is thought to involve a combination of genetic susceptibility and environmental factors, though the full picture isn’t completely understood.
Other causes include surgical removal of the thyroid (often for cancer or nodules), radiation therapy to the head or neck, and certain medications that interfere with hormone production. In parts of the world where diets lack sufficient iodine, deficiency remains a leading cause, since iodine is a building block the thyroid needs to manufacture its hormones. Iron deficiency can also impair thyroid function by disrupting the enzyme that incorporates iodine into thyroid hormones.
Recognizing the Symptoms
Hypothyroidism tends to develop slowly, so symptoms often creep in over months and are easy to dismiss as normal aging or stress. The most common signs include:
- Fatigue that doesn’t improve with rest
- Unexplained weight gain or difficulty losing weight
- Cold intolerance, feeling chilled when others are comfortable
- Dry skin and dry, thinning hair
- Joint and muscle pain, stiffness, or cramping
- Constipation
- Depression or low mood
- Slowed heart rate
- Heavy or irregular menstrual periods and fertility problems
No single symptom points definitively to hypothyroidism. Many of these overlap with other conditions. What makes the pattern distinctive is the combination of several symptoms appearing together, all pointing toward a body that’s running too slowly.
How It’s Diagnosed
Diagnosis is straightforward and relies on a simple blood test measuring TSH, or thyroid-stimulating hormone. TSH is produced by your pituitary gland to tell the thyroid how much hormone to make. When the thyroid isn’t producing enough, the pituitary pumps out more TSH to compensate, so a high TSH level is the earliest signal of an underactive thyroid. The normal adult range for TSH is roughly 0.4 to 4.2 mU/L, though exact cutoffs vary slightly between labs.
Hypothyroidism progresses through two stages. In the early, subclinical stage, TSH is mildly elevated but the actual thyroid hormone levels (free T4 and T3) remain normal. You may have few or no noticeable symptoms at this point. In overt hypothyroidism, TSH is clearly elevated and free T4 and T3 have dropped below their normal ranges, which is when symptoms typically become more pronounced. Your doctor may also test for thyroid antibodies to determine whether Hashimoto’s is the underlying cause.
When Treatment Starts
Not every elevated TSH reading requires immediate medication. A significant proportion of people with mildly elevated levels will see their TSH return to normal on its own, so the initial approach is often to recheck levels in two to three months to confirm the elevation is sustained.
Both the American Thyroid Association and the American Association of Clinical Endocrinology recommend starting thyroid hormone replacement when TSH rises above 10 mU/L. Treatment may also begin at lower TSH levels if you have thyroid antibodies, noticeable symptoms, or cardiovascular risk factors. For adults over 70, treatment decisions are more cautious because the risks and benefits shift with age.
What Treatment Looks Like
The standard treatment is a daily synthetic thyroid hormone tablet called levothyroxine, which replaces the hormone your thyroid can no longer make in sufficient quantities. For most people, this single pill taken once a day effectively resolves symptoms.
Timing and absorption matter more than you might expect. You should take the tablet on an empty stomach, half an hour to a full hour before breakfast. Calcium supplements, iron supplements, and antacids can block absorption, so you need at least a four-hour gap between these and your thyroid medication. High-fiber foods, soy products, and walnuts can also reduce how much of the drug your body absorbs. Even grapefruit juice can delay absorption. Stomach acid plays a role too, so medications that reduce acid (like proton pump inhibitors) may interfere.
After starting treatment, your TSH levels will be rechecked periodically, and the dose adjusted until your levels stabilize within the normal range. Most people take levothyroxine for life, since the underlying thyroid damage is typically permanent. The good news is that once you find the right dose, the medication is well tolerated and side effects are uncommon.
Nutrients That Support Thyroid Function
Three trace minerals are essential for your thyroid to produce and activate its hormones. Iodine is the raw material the gland uses to build T3 and T4. In most developed countries, iodized salt provides enough, but people on very restricted diets can fall short. Selenium protects thyroid cells from oxidative damage during hormone production and plays a role in converting T4 into the more active T3. The recommended selenium intake for thyroid health is 150 to 200 micrograms per day, which you can get from foods like Brazil nuts, seafood, and eggs. Iron is needed for the enzyme that incorporates iodine into thyroid hormones. Iron deficiency and anemia are associated with hypothyroidism, making adequate iron intake especially important for people already at risk.
Risks of Leaving It Untreated
Mild hypothyroidism progresses slowly, but ignoring it over the long term carries real consequences. Chronically low thyroid hormone levels raise LDL cholesterol and triglycerides, increasing your risk of heart disease. Reduced cardiac output and conduction abnormalities can develop as the heart muscle weakens from prolonged hormone deficiency.
The most dangerous complication is myxedema coma, a rare but life-threatening emergency that can develop when severe hypothyroidism is left untreated for a long time and then pushed over the edge by a stressor like an infection, surgery, or extreme cold. It involves a dramatic drop in body temperature, loss of consciousness, and dysfunction across multiple organ systems, including the heart, lungs, kidneys, and brain. Cardiac involvement can lead to heart failure, dangerous rhythm disturbances, and shock. Mortality rates are high, but the condition is almost entirely preventable with routine thyroid treatment. Timely hormone replacement can often reverse even significant cardiac changes before they become permanent.