What is TSH? Thyroid-Stimulating Hormone Explained, Levels and Function

What is TSH and Why Does Your Body Need It?

TSH, or thyroid-stimulating hormone, controls how the thyroid gland functions. It signals the thyroid when to produce more hormones and when to reduce output. Thyroid hormones regulate metabolism, energy levels, body temperature, heart rate, brain function and the activity of many organs. By adjusting hormone production according to the body’s needs, TSH helps maintain hormonal balance.

Where is TSH Produced?

TSH is produced by the pituitary gland, a small gland located at the base of the brain. The pituitary releases TSH in response to signals from the hypothalamus. Once released into the bloodstream, TSH travels to the thyroid gland in the neck, where it signals the thyroid to increase or decrease hormone production based on the body’s needs.

What Happens When TSH Levels Are Too Low?

Low TSH levels usually mean the thyroid is producing more hormones than the body needs. This condition is called hyperthyroidism and occurs when the thyroid becomes overactive. TSH levels below 0.4 mU/L are commonly associated with this state, as excess thyroid hormones suppress TSH release from the pituitary gland. In rare cases, problems with the pituitary gland itself can also lead to low TSH.

Common Symptoms of Low TSH (Hyperthyroidism):

• Rapid or irregular heartbeat (palpitations)
• Feeling anxious, restless or shaky
• Unintended weight loss despite a good appetite
• Increased bowel movements
• Enlarged thyroid (goitre), causing neck swelling
• Eye changes, such as bulging in some cases
• Warm, thin or moist skin
• Irregular menstrual cycles

What Happens When TSH Levels Are Mildly High?

TSH levels between 4 and 10 mU/L usually indicate mild (subclinical) hypothyroidism, meaning the thyroid is producing slightly less hormone than the body needs. This rise in TSH shows that the pituitary gland is working harder to stimulate thyroid hormone production. Common causes include early autoimmune thyroid disease, iodine deficiency or temporary thyroid inflammation.

Symptoms of Mild Hypothyroidism May Include:

• Feeling slightly tired or low on energy
• Mild weight gain without changes in diet or activity
• Feeling colder than usual
• Dry or brittle hair and skin
• Mild mood changes or low mood
• Small changes in menstrual cycles

What Happens When TSH Levels Are Low?

Low TSH levels usually indicate that the thyroid is producing more hormones than the body needs. This condition is called hyperthyroidism, or an overactive thyroid. When thyroid hormone levels are high, the pituitary gland reduces TSH release because further stimulation is not needed. As a result, many body functions speed up, affecting the heart, energy levels and temperature regulation.

Symptoms of Low TSH (Hyperthyroidism) May Include:

• Feeling anxious, restless or unusually jittery
• Fast or irregular heartbeat (palpitations)
• Feeling overheated or sweating more than usual
• Unintended weight loss despite a normal or increased appetite
• Difficulty sleeping or feeling constantly alert

Can Abnormal TSH Affect Fertility?

Yes. Abnormal TSH levels can interfere with reproductive hormone function. In women, high or low TSH may disrupt the menstrual cycle and ovulation, leading to irregular or absent periods. Thyroid imbalance can also affect embryo implantation and early pregnancy development.

In men, abnormal TSH can impair sperm quality, including sperm count and motility. Even mild thyroid dysfunction may have an impact. Because of this, TSH testing is commonly included in fertility evaluations for both women and men.

How is TSH Tested and When Should You Consider It?

TSH is measured through a blood test, usually without the need for fasting. Blood is drawn from a vein in the arm and sent to a laboratory, with results typically available within one or two days. Doctors often check Free T3 and Free T4 alongside TSH to assess overall thyroid function.

A TSH test is recommended if symptoms persist, such as irregular menstrual cycles, unexplained fatigue or difficulty getting pregnant or planning a pregnancy. Testing is also advised if there is a family history of thyroid disorders.

How Are TSH Levels Managed?

Knowing what is TSH helps explain how treatment works. TSH shows how hard the body is trying to control the thyroid. Treatment focuses on fixing that imbalance.

Management usually includes:

• Thyroid Hormone Replacement: Given when TSH is high and the thyroid is underactive. The medication replaces the hormone the thyroid is not producing. Blood tests are used to adjust the dose over time.
• Medicines That Slow Thyroid Hormone Production: Used when TSH is low because the thyroid is overactive. These medicines reduce hormone output and help ease symptoms such as a fast heartbeat or nervousness.
• Regular Blood Tests: TSH levels are checked every few months, especially at the start of treatment. Medication doses are adjusted if levels move outside the target range.

Conclusion

Understanding TSH helps you recognise when subtle symptoms may be linked to a thyroid imbalance rather than everyday stress or lifestyle changes. Because thyroid issues often develop slowly, they are easy to overlook without testing.

A simple TSH blood test can provide valuable insight, especially if symptoms persist or you are planning a pregnancy. With early detection and regular follow-up, most thyroid conditions can be managed effectively, allowing you to take informed steps toward better hormonal and overall health.