Low thyroid, or hypothyroidism, means your thyroid gland isn’t producing enough hormones to keep your body’s metabolism running at its normal pace. The normal range for TSH, the main blood marker used to check thyroid function, is roughly 0.4 to 4.0 mIU/L. When your thyroid falls behind, TSH climbs above that range as your brain tries to push the gland to work harder. The result is a body-wide slowdown that can affect everything from your energy levels to your heart rate.
How Your Thyroid Controls Metabolism
Your thyroid is a small, butterfly-shaped gland at the front of your neck. Its job is to release hormones (T3 and T4) that set the pace for nearly every cell in your body. These hormones increase oxygen consumption, energy use, and body temperature. When levels are normal, you don’t notice any of this. It just runs in the background like a thermostat.
The system is controlled by a feedback loop. Your brain monitors how much thyroid hormone is circulating and releases TSH (thyroid-stimulating hormone) to tell the gland to produce more or less. When thyroid hormone drops too low, TSH rises. That’s why a high TSH on a blood test is the hallmark sign of an underactive thyroid. It means your brain is shouting at the gland to pick up the pace, but the gland can’t keep up.
What It Feels Like
Hypothyroidism tends to creep in gradually. Early on, you might just feel more tired than usual or notice some unexplained weight gain. Because these symptoms are so common and nonspecific, many people chalk them up to stress or aging. But as thyroid hormone levels continue to drop, the signs become harder to ignore:
- Fatigue and sluggishness that doesn’t improve with rest
- Weight gain that’s hard to explain through diet alone
- Feeling cold when others around you are comfortable
- Dry, coarse skin and thinning hair
- Muscle aches, stiffness, or weakness
- A puffy face, particularly around the eyes
- A hoarse voice
- Heavier or irregular menstrual periods
- Depression and memory problems
- A slower-than-normal heart rate
Children and teens can develop hypothyroidism too. Along with the symptoms above, they may experience delayed growth, late puberty, and slower mental development. Newborns born with a thyroid that doesn’t work properly often show no symptoms at first, which is why hospitals screen for the condition at birth.
What Causes It
The most common cause in countries with adequate iodine intake is an autoimmune condition called Hashimoto’s thyroiditis. In Hashimoto’s, the immune system gradually attacks the thyroid gland, reducing its ability to make hormones over time. About 15% to 25% of the general population tests positive for the antibodies associated with Hashimoto’s, though not all of them go on to develop full-blown hypothyroidism. Women and older adults are affected more often.
Other causes include surgical removal of part or all of the thyroid (often for nodules or cancer), radiation treatment to the head or neck, and certain medications. Worldwide, iodine deficiency remains a major cause, and even excessive iodine intake has been linked to up to a fourfold increase in Hashimoto’s incidence. Temporary hypothyroidism can also occur after pregnancy or during periods of thyroid inflammation.
Subclinical vs. Overt Hypothyroidism
Not every case of low thyroid is the same severity. Doctors distinguish between two stages. Overt hypothyroidism means your TSH is elevated and your T4 is clearly low. This is the full-blown version with noticeable symptoms. Subclinical hypothyroidism is an earlier, milder stage where TSH is elevated but T4 is still within the normal range. You may have mild symptoms or none at all.
Subclinical hypothyroidism is quite common, and it doesn’t always need treatment. If your TSH is under 10 mIU/L with a normal T4, your doctor will typically recheck your levels in three to six months, because nearly half of people in this range see their numbers return to normal on their own. Treatment is generally considered when TSH rises above 10, when thyroid antibodies are present, or when you have clear symptoms or cardiovascular risk factors.
Why It Matters for Heart Health
Left untreated, hypothyroidism does more than make you tired. It directly raises LDL (“bad”) cholesterol and total cholesterol levels. In fact, an underactive thyroid is one of the more common causes of high cholesterol that isn’t related to diet or genetics. Over time, this unfavorable shift in blood lipids increases the risk of heart disease. Treating the thyroid problem often improves cholesterol numbers without any additional medication.
Untreated hypothyroidism can also lead to an enlarged thyroid gland (goiter), which in severe cases may cause difficulty swallowing or breathing. In rare, extreme situations, profoundly low thyroid hormone levels can become a medical emergency.
How It’s Treated
Treatment is straightforward: a daily pill of synthetic thyroid hormone (levothyroxine) that replaces what your body isn’t making. The standard starting dose for adults with overt hypothyroidism is based on body weight, typically around 1.6 micrograms per kilogram. Older adults usually start at a lower dose. Your doctor will check your TSH every six to eight weeks initially and adjust the dose until your levels stabilize in the normal range.
Most people feel noticeably better within a few weeks of starting treatment, though it can take a couple of months for symptoms to fully resolve. The medication is taken for life in most cases, since the underlying gland damage usually doesn’t reverse.
Getting the Most From Your Medication
Levothyroxine is sensitive to what else is in your stomach. A number of common foods and supplements interfere with how well the pill is absorbed. To get the full dose, take it on an empty stomach, ideally first thing in the morning. Then wait at least an hour before having coffee, fiber-rich foods, or soy products. If you take calcium or iron supplements, space them two to four hours after your thyroid pill. Even cow’s milk and fruit juices can reduce absorption if consumed too soon.
Consistency matters more than perfection here. Taking your medication at the same time each day, with the same routine around food, keeps your hormone levels steady and makes it easier for your doctor to dial in the right dose.
Thyroid Levels During Pregnancy
Pregnancy places extra demand on the thyroid, and the acceptable TSH range is tighter than it is for the general population. During the first trimester, many guidelines set the upper limit of normal at 2.5 mIU/L, compared to 4.0 mIU/L outside of pregnancy. The second trimester target rises slightly to 3.0, and the third to 3.5. Women who already take thyroid medication often need a dose increase early in pregnancy, sometimes by 30% to 50%. Untreated hypothyroidism during pregnancy raises the risk of complications for both the mother and the developing baby, so thyroid levels are monitored closely throughout.