Why thyroid levels can sometimes vary from person to person

If your thyroid isn’t working well, you could gain or lose weight and experience mood shifts or heart palpitations. Or you may not have any symptoms at all.

A blood test can show whether your thyroid is working properly by measuring the amount of thyroid stimulating hormone (TSH) in your body. TSH, which the pituitary gland produces, causes the thyroid to secrete thyroid hormone. That hormone regulates your metabolism, mood and heart rate, among other functions in the body. The normal range for a person’s TSH is 0.5 to 4.5 mIU/L (milli-international units per liter).

If you’re not in that normal range, that could be a sign of a problem. Acting like a thermostat, your pituitary gland releases TSH at below the normal range when your thyroid is overactive and producing too much thyroid hormone. When your thyroid isn’t making enough thyroid hormone, TSH levels go above the normal range.

But at certain times, your TSH level may be outside of the normal range of 0.5 to 4.5 mIU/L and it’s not problematic, such as when you’re a child or elderly, pregnant or have thyroid cancer.

What can cause lower levels of TSH without being a problem

If you’re pregnant, extra thyroid hormone is produced in your body because a fetus depends on the mother’s thyroid hormone to develop organs in the first 12 to 14 weeks. As a result, during pregnancy, your TSH levels will be below average.

If you have thyroid cancer, your doctor may want to keep your TSH levels very low. A mildly low level of TSH is considered beneficial if you have thyroid cancer because it’s believed to help slow the growth of cancer.

What can cause higher levels of TSH without being a problem

In children, TSH levels are highest at birth and gradually go down as a child gets older. As a result, the TSH levels among children can vary.

At the other end of the life cycle, if you’re 70 or older, you might have TSH levels that are higher than the normal range. That could be because your thyroid is slowing down in producing hormones. With less thyroid hormone in your body, your metabolism decelerates as well, putting less strain on your heart, which may be beneficial when you’re older.

Symptoms of an overactive or underactive thyroid

Thyroid problems are more common in women than in men. The most common thyroid disease in both women and men is an underactive thyroid also known as hypothyroidism. That happens when the thyroid gland isn’t producing enough hormone.

It’s possible you could have an underactive thyroid and not feel any symptoms if your TSH level is below 10 mIU/L. But if your TSH is over 10 mIU/L, most people will feel symptoms including weakness, sluggishness, cold intolerance, constipation and dry skin. An underactive thyroid can also slow your heart rate and increase your blood pressure and cholesterol levels.

If, on the other hand, your thyroid gland is producing too much hormone — also called hyperthyroidism — your TSH levels drop below average, and you may experience symptoms such as your heart racing or skipping beats, heat intolerance, inability to sleep, frequent bowel movements and irritability or anxiety. Hyperthyroidism can bring about an abnormal heart rhythm, osteoporosis and bone fractures, and it can also cause infertility and an increased risk of miscarriage.

More about signs that something could be wrong with your thyroid

What you can do about your thyroid concerns

Whether you experience a thyroid problem has a lot to do with whether your parents or close relatives have or had one. Tell your primary care provider any concerns you have about sudden weight changes that come with shifts in your energy level or moods. A simple blood test of your TSH could help you learn what’s going on and what your options are for feeling better.