An underactive thyroid, known medically as hypothyroidism, is a condition where your thyroid gland doesn’t produce enough hormones to keep your body running at its normal pace. It affects roughly 5% of adults and is far more common in women, especially after age 60. The result is a widespread slowdown: your metabolism drops, your energy fades, and dozens of body processes that depend on thyroid hormones start to lag.
What Your Thyroid Actually Does
Your thyroid is a small, butterfly-shaped gland at the front of your neck. Its main job is producing hormones that regulate how fast your cells burn energy. These hormones influence nearly every organ in your body. They set your baseline metabolic rate, help your liver clear cholesterol from the blood, control how fast your heart beats, and even affect how quickly you think and process information.
The system works like a thermostat. Your pituitary gland, a pea-sized structure at the base of your brain, monitors thyroid hormone levels in your blood. When levels drop too low, the pituitary releases a signal called TSH (thyroid-stimulating hormone) telling the thyroid to produce more. When levels are adequate, the pituitary dials TSH back down. In hypothyroidism, this feedback loop is disrupted. Either the thyroid can’t respond to the signal, or the pituitary itself isn’t sending the right message.
Common Symptoms
Because thyroid hormones touch so many systems, the symptoms of an underactive thyroid are broad and often develop gradually over months or years. The most frequently reported include:
- Fatigue that doesn’t improve with rest
- Unexplained weight gain or difficulty losing weight
- Cold intolerance, feeling chilly when others are comfortable
- Joint and muscle pain
- Dry skin or dry, thinning hair
- Heavy or irregular periods and fertility problems
- Slowed heart rate
- Depression or low mood
Many of these, particularly fatigue and weight gain, are extremely common in the general population and don’t automatically point to a thyroid problem. What makes hypothyroidism more likely is experiencing several of these symptoms together, especially if they developed around the same time or have been worsening. Constipation, a puffy face, brain fog, and a hoarse voice are also typical but less well-known signs.
Why It Happens
The most common cause by far is Hashimoto’s thyroiditis, an autoimmune condition where your immune system mistakenly attacks the thyroid gland. Over time, this assault destroys enough thyroid tissue that the gland can no longer keep up with hormone demand. Hashimoto’s tends to run in families and is diagnosed through blood tests that detect specific antibodies (thyroid peroxidase antibodies being the most commonly checked).
Other causes of primary hypothyroidism, where the thyroid itself is the problem, include surgical removal of the thyroid (often for cancer or large goiters), radioactive iodine treatment used for an overactive thyroid, certain medications, and radiation therapy to the head or neck. Iodine deficiency is a major cause worldwide but is uncommon in countries where salt is iodized.
In rare cases, the problem isn’t the thyroid gland at all. Central hypothyroidism occurs when the pituitary gland or the brain region above it (the hypothalamus) fails to send the right signals. This can result from pituitary tumors, head trauma, brain surgery, or cranial radiation. These cases are harder to catch on standard screening because the usual blood marker, TSH, may look misleadingly normal even though thyroid hormone levels are low.
How It Affects Your Heart and Cholesterol
Left untreated, low thyroid hormone levels create real cardiovascular risk. Insufficient thyroid hormone slows your heart rate and makes your arteries less elastic, which forces blood pressure upward to keep blood circulating. At the same time, your liver’s ability to process cholesterol slows down. This raises LDL (“bad”) cholesterol and contributes to narrower, hardened arteries over time.
These effects aren’t just theoretical risks for people with severe disease. Even mildly low thyroid function, sustained over years, can meaningfully shift your cholesterol profile and blood pressure. This is one reason doctors take hypothyroidism seriously even when symptoms seem manageable.
Getting Diagnosed
Diagnosis starts with a simple blood test measuring TSH. When the thyroid is underperforming, the pituitary pumps out more TSH in an attempt to compensate, so a high TSH level is the classic red flag. A second test measuring free T4 (the main hormone the thyroid produces) confirms whether your thyroid is actually falling short.
There’s also a gray zone called subclinical hypothyroidism, where TSH is elevated but T4 remains in the normal range. You may have mild symptoms or none at all. For most people in this category, doctors take a “wait and see” approach with periodic re-testing rather than starting medication right away. Treatment is more commonly recommended when TSH rises above 10 mIU/L, when you’re young or middle-aged with symptoms, or when you’re trying to conceive. For women undergoing fertility treatment, guidelines recommend bringing TSH below 2.5 mIU/L.
Treatment and What to Expect
Treatment is straightforward: a daily pill of synthetic thyroid hormone (levothyroxine) that replaces what your body isn’t making. It’s one of the most commonly prescribed medications in the world. Current guidelines base the starting dose on your body weight, typically around 1.6 micrograms per kilogram per day using lean body mass. People who are significantly overweight may need a lower weight-based dose to avoid overtreatment.
Once you start, it takes about six to eight weeks for your levels to stabilize enough to recheck. Your doctor will test your TSH and adjust the dose as needed. Most people require at least one or two adjustments before landing on the right amount. After that, you’ll typically have your levels checked once or twice a year.
Most people feel noticeably better within a few weeks of starting treatment, though some symptoms like hair thinning and dry skin can take several months to fully resolve. The medication is a lifelong commitment for the vast majority of people with hypothyroidism, since the underlying cause (usually autoimmune damage) doesn’t reverse.
Getting the Most From Your Medication
Levothyroxine is absorbed in the gut, and several common substances can interfere with that process. Calcium supplements, iron supplements, and antacids are the most well-known culprits. Standard guidance is to take the medication on an empty stomach, ideally 30 to 60 minutes before eating.
Coffee has long been considered a concern, but newer research suggests that liquid formulations of the medication aren’t affected by coffee consumed shortly after the dose. If you take the standard tablet form, however, the traditional advice to wait before drinking coffee still applies. The simplest strategy for many people is to take the pill first thing in the morning and eat breakfast after getting ready, which naturally builds in a 30 to 60 minute gap without much effort.
Consistency matters more than perfection. Taking your medication at roughly the same time each day, with the same relationship to food, gives your body a steady supply and makes blood test results more reliable when it’s time to check your dose.