The thyroid is a butterfly-shaped, hormone-secreting gland located in the front of the neck. Thyroid hormones play an important role in:

  • metabolism and weight
  • body temperature
  • mood
  • muscle control
  • digestion
  • growth
  • brain function and development
  • heart function

Several conditions can cause a thyroid hormone imbalance. The most common are autoimmune diseases and iodine deficiency.

If your doctor suspects that you have a thyroid disorder, they may recommend one or several blood tests that are part of a thyroid function panel. These tests measure thyroid hormone levels in the blood and can help determine how your thyroid gland is functioning.

The most common tests are:

  • thyroid-stimulating hormone (TSH)
  • free T4

In some cases, your doctor may also perform the following tests:

  • free T3
  • calcitonin
  • thyroglobulin
  • thyroid antibodies

The TSH test is sometimes given on its own as a preliminary screening test. TSH is produced in the pituitary gland. It stimulates the thyroid to produce triiodothyronine (T3) and thyroxine (T4).

An imbalance in TSH provides information about your thyroid and its ability to produce and secrete thyroid hormones. It’s often the most sensitive indicator that a thyroid problem is present.

The normal range of TSH levels in adults is between 0.4 to 4.0 mIU/L (milli-international units per liter). Some research suggests that this range should actually be more like 0.45 to 2.5 mIU/L.

The TSH range may also vary slightly based on the testing facility where your blood is being analyzed.

TSH levels in children, babies, and pregnant women can fall outside of the normal range for adults.

If you’re already being treated for a thyroid disorder, your doctor will probably consider your TSH level to be normal if it’s somewhere between 0.5 to 3.0 mIU/L. This may vary based upon your age and gender.

TSH levels are only one indicator of how your thyroid is functioning. They vary by gender, age, and other factors. In general, normal, low, and high TSH levels are:

GenderAgeNormalLowHigh
Male18-300.5-4.15 mIU/L<0.5 mIU/L>4.5 mIU/L
Male31-500.5-4.15 mIU/L<0.5 mIU/L>4.15 mIU/L
Male51-700.5-4.59 mIU/L<0.5 mIU/L>4.6 mIU/L
Male71-900.4-5.49 mIU/L<0.4 mIU/L>5.5 mIU/L
Female18-290.4-2.34 mIU/L<0.4 mIU/L>4.5 mIU/L
Female30-490.4-4.0 mIU/L<0.4 mIU/L>4.1 mIU/L
Female50-790.46-4.68 mIU/L<0.46 mIU/L4.7-7.0 mIU/L

Thyroid hormones can affect brain and nervous system development of the baby, especially during the first trimester. At around 12 weeks, the baby will start to produce thyroid hormones of its own. Until then, the baby is entirely dependent on the transfer of thyroid hormones from the mother.

Both hypothyroidism and hyperthyroidism can occur during pregnancy. You may also have one of these conditions prior to getting pregnant and not know it.

Untreated thyroid disease can cause miscarriage, premature birth, or low birth weight. It can also cause preeclampsia. Having an underactive thyroid during pregnancy can also affect the baby’s growth and brain development.

It’s important to have your TSH levels checked during pregnancy to make sure that both you and your baby remain healthy.

Hormones secreted during pregnancy can affect TSH levels, altering them from your typical numbers.

This chart provides an overview of normal, low, and high TSH levels for pregnant women who are between 18 and 45 years old:

NormalLowHigh
First Trimester0.2-2.5 mIU/L<0.2 mIU/L2.5-10 mIU/L
Second Trimester0.3-3.0 mIU/L<0.3 mIU/L3.01-4.50 mIU/L
Third Trimester0.8-5.2 mIU/L <0.8 mIU/L>5.3 mIU/L

Talk to your doctor about your TSH levels during pregnancy. Your doctor can determine if you need thyroid medication or if your existing thyroid medication needs to be adjusted based on the desired TSH levels for the different stages of pregnancy.

If your TSH level is lower than it should be, you may have hyperthyroidism. This occurs when the pituitary gland senses thyroid hormone levels that are too high and compensates by decreasing TSH production.

Complications of hyperthyroidism include:

Symptoms of low TSH levels

Many symptoms of hyperthyroidism are also caused by other conditions. A doctor can determine if the symptoms you’re experiencing are caused by low TSH levels or something else. Older adults may have little to no symptoms.

Some symptoms to watch out for include:

  • rapid or irregular heartbeat
  • heart palpitations (pounding heart)
  • unexplained weight loss
  • feeling agitated or nervous
  • tremors in the hands and fingers
  • fatigue or exhaustion
  • feeling hungry more often than usual
  • insomnia
  • thinning of the skin or hair
  • change in bowel movements, especially greater rates of frequency
  • increased sweating
  • changes in menstrual cycle

Causes of an overactive thyroid

An overactive thyroid can be caused by several conditions, including:

If your TSH level is higher than it should be, you may have hypothyroidism. This occurs when the pituitary gland overcompensates for low amounts of thyroid hormone by pumping out more TSH.

This condition is most common in older women, but it can occur in any gender at any age. Infants with congenital hypothyroidism can also be born with high TSH levels. The risks and complications of underactive thyroid vary based on age.

If left untreated, risks and complications of hypothyroidism in adults can include:

If left untreated, risks and complications in infants include:

If left untreated, risks and complications in children and teenagers include:

  • growth delay, causing shortness of height
  • delayed puberty
  • delayed growth of permanent teeth
  • developmental delays and reduced cognitive ability

Symptoms of high TSH levels

Hypothyroidism can be asymptomatic in its early stages. As it progresses, you may experience some or all of these symptoms below.

Many of the symptoms are nonspecific and can be seen in other conditions as well. So, it’s important to have your thyroid hormones tested if you experience any of the following:

  • memory problems
  • goiter
  • slowed heart rate
  • depression
  • weight gain
  • swollen, stiff, or painful joints
  • fatigue
  • constipation
  • dry skin or hair
  • thinning hair
  • changes in menstruation
  • increased sensitivity to cold

Causes of an underactive thyroid

An underactive thyroid can be caused by:

Having a doctor monitor your thyroid gland and its production of thyroid hormones is the only way to comprehensively determine thyroid health.

The thyroid is one of several glands and structures that make up the body’s endocrine system.

It works in partnership with the pituitary and hypothalamus glands. These glands stimulate the thyroid to secrete two hormones, which are then released into the bloodstream: T4 and T3.

If your thyroid gland doesn’t produce enough T3 or T4, hypothyroidism (underactive thyroid) can result. If your thyroid gland produces too much T4, hyperthyroidism (overactive thyroid) can result.

It’s important to note that there’s some controversy about what TSH levels are normal. For this reason, it’s important to talk to your doctor about any symptoms you’re experiencing. You should also let them know about other medical conditions you have and any medications you’re taking.

A doctor will diagnose a thyroid disorder by reviewing your medical history, conducting a physical examination, and doing multiple tests, including a blood test. In some, but not all, instances, you may need a thyroid ultrasound or thyroid scan.

Treatments for hypothyroidism and hyperthyroidism can change over time, based on the severity of the condition and your response to medication.

Hypothyroidism (high TSH)

Hypothyroidism is treated with levothyroxine (Synthroid), a synthetic thyroid hormone. Levothyroxine is an oral medication that’s taken daily on an empty stomach. Your dosage may change over time and is usually adjusted based on blood levels.

Hyperthyroidism (low TSH)

Hyperthyroidism may be treated several ways. Your age, the severity of the condition, the underlying cause, and your overall health will be taken into account. Treatments include:

  • Anti-thyroid medications. These oral medications stop your thyroid from producing too much hormone. The most commonly prescribed medication for this condition is methimazole (Tapazole).
  • Radioactive iodine. This is a pill, taken orally, which is designed to destroy some or all of the thyroid. It’s usually needed only once, although some people may need more than one treatment. Eventually, you may need to take daily thyroid replacement medication.
  • Thyroidectomy. This surgical procedure removes most of your thyroid gland. It’s followed up with daily thyroid hormone replacement medication.

Thyroid level tests, including the TSH test, can determine if your thyroid is producing too much or too little thyroid hormone.

If your thyroid gland doesn’t produce enough T3 or T4, hypothyroidism (underactive thyroid) can result. If your thyroid gland produces too much T4, hyperthyroidism (overactive thyroid) can result.

Imbalances in TSH levels are common and treatable.