TSH, or Thyroid Stimulating Hormone, is produced by the pituitary gland at the base of your brain. While the term “fTSH” is sometimes mistakenly used, TSH is not typically described as “free” because its measurement reflects the total circulating hormone. TSH signals the thyroid gland, a butterfly-shaped organ in your neck, to produce and release its hormones. This hormone regulates thyroid function, influencing numerous bodily processes.
The Thyroid’s Master Regulator
TSH is a central part of the endocrine system’s feedback loop. The hypothalamus in the brain initiates this process by releasing thyrotropin-releasing hormone (TRH), which stimulates the pituitary gland to produce TSH. TSH then travels through the bloodstream to the thyroid gland, prompting it to synthesize and release thyroid hormones, primarily thyroxine (T4) and triiodothyronine (T3).
Thyroid hormones are essential for maintaining the body’s metabolic rate, which is the speed at which your body converts food into energy. T4 is the most abundant thyroid hormone produced, serving as a prohormone that is later converted into the more active T3 in various tissues like the liver and kidneys. These hormones regulate heart and digestive functions, muscle control, brain development, and bone maintenance. They also influence body temperature and control how quickly your body replaces dying cells.
Interpreting TSH Levels
A TSH blood test is a primary tool for assessing thyroid function, measuring the amount of this hormone in your bloodstream. A normal TSH range falls between 0.4 and 4.0 milliunits per liter (mU/L), though specific laboratory ranges can vary. Deviations from this range can signal an imbalance in thyroid hormone production.
High TSH levels indicate an underactive thyroid, known as hypothyroidism. Here, the pituitary gland produces more TSH to stimulate a sluggish thyroid to produce sufficient thyroid hormones. Hashimoto’s disease, an autoimmune condition where the immune system attacks the thyroid gland, is a common cause.
Conversely, low TSH levels suggest an overactive thyroid, or hyperthyroidism. Here, the thyroid produces an excess of hormones, signaling the pituitary to reduce TSH output. Graves’ disease, another autoimmune disorder, is a frequent cause, characterized by antibodies that stimulate the thyroid to overproduce hormones.
Factors Influencing TSH Results
TSH levels can be influenced by various factors beyond underlying thyroid conditions. Age plays a role, as TSH ranges tend to increase with age. Pregnancy also impacts TSH levels, with different normal ranges observed across trimesters.
Certain medications can affect TSH test results. Biotin supplements may lead to falsely low TSH readings, so it’s advisable to stop their intake for at least two days before a test. Other medications, including corticosteroids, dopamine agonists, lithium, and some antidepressants, can alter TSH levels by affecting hormone production or metabolism. Acute illnesses or significant stress on the endocrine system can also temporarily suppress TSH levels.
Beyond the TSH Test
An abnormal TSH result serves as an initial indicator, prompting further diagnostic steps to understand thyroid function. Following an unusual TSH reading, healthcare providers order additional blood tests for comprehensive assessment. These include measuring free T4 (fT4) and free T3 (fT3) levels.
Free T4 and free T3 measure the unbound, active forms of thyroid hormones available to body tissues, providing a clearer picture of hormone activity than total T4 or T3 measurements. If an autoimmune condition like Hashimoto’s or Graves’ disease is suspected, thyroid antibody tests, such as thyroid peroxidase antibodies (TPOAb) and thyrotropin receptor antibodies (TRAb), are performed. These additional tests help pinpoint the cause of the TSH imbalance, guiding management and treatment strategies.