TSH Test

TSH (thyroid-stimulating hormone) is a hormone made by your pituitary gland—a small organ at the base of your brain that acts as the control center for many other glands. TSH’s main job is to tell your thyroid gland, located in your neck, how much hormone to release. Specifically, it signals the thyroid to produce T4 (thyroxine), which your body can convert into the more active hormone T3 (triiodothyronine). These thyroid hormones help regulate how your body uses energy, which affects everything from heart rate and body temperature to digestion and mood.

What makes TSH unique is how tightly it’s regulated through a feedback loop. When your thyroid hormone levels (especially free T4) are too low, the pituitary gland increases TSH to boost production. If T4 levels get too high, TSH is dialed down to bring things back into balance. This inverse relationship makes TSH a very sensitive marker—it often changes before T4 or T3 levels do, which is why it’s usually the first test checked to evaluate thyroid health.

When TSH is high, it usually means your thyroid is underactive (hypothyroidism). In this state, your pituitary is working overtime, trying to coax more hormone out of a sluggish thyroid. People with high TSH often feel tired, gain weight easily, feel cold, or have constipation, depression, or dry skin. The most common cause is Hashimoto’s thyroiditis, an autoimmune condition that damages the thyroid over time.

On the flip side, low TSH typically signals an overactive thyroid (hyperthyroidism), where too much thyroid hormone is being produced. Here, the pituitary pulls back on TSH in an attempt to slow things down. Symptoms may include anxiety, insomnia, weight loss, rapid heart rate, heat intolerance, and sweating. Graves’ disease is a common trigger, where the immune system over-stimulates the thyroid.

However, TSH doesn’t always tell the full story. In central hypothyroidism, which is caused by a problem in the pituitary or hypothalamus (not the thyroid), TSH can be low or normal even when thyroid hormone levels are dangerously low. In these cases, relying on TSH alone can miss the diagnosis, which is why free T4 should always be checked if central thyroid issues are suspected—such as in someone with fatigue and menstrual changes, or a known pituitary tumor.

Please note that biotin supplements can also interfere with lab tests, especially at higher doses. If you’re taking biotin for hair or nail health, it can cause falsely low TSH readings and misleadingly high free T4 levels, mimicking hyperthyroidism. It’s best to stop biotin at least two days before testing.