The big picture: TSH (Thyroid-Stimulating Hormone) is produced by the pituitary gland and plays a central role in regulating thyroid hormone production. It stimulates the thyroid gland to release thyroxine (T4) and triiodothyronine (T3), which are crucial for maintaining metabolism, energy levels, and various bodily functions. Changes in TSH can serve as an “early warning system” of thyroid dysfunction often occurring before the actual level of thyroid hormones becomes too high or too low.

How TSH regulates thyroid function: The pituitary gland continuously monitors circulating thyroid hormone levels. When these levels drop, the pituitary releases more TSH to stimulate the thyroid to produce more hormones. Conversely, when thyroid hormone levels are too high, the pituitary reduces TSH production to lower thyroid activity, maintaining hormonal balance.

High TSH (hypothyroidism): High TSH levels typically indicate an underactive thyroid (hypothyroidism), where the thyroid is not producing enough hormones. Common symptoms include fatigue, weight gain, cold intolerance, depression, dry skin, and constipation. The most frequent cause is Hashimoto’s thyroiditis. Treatment involves thyroid hormone replacement therapy, commonly using levothyroxine, to restore normal thyroid function and bring TSH levels down.

Low TSH (hyperthyroidism): Low TSH levels suggest an overactive thyroid (hyperthyroidism), where excessive thyroid hormones are produced. Symptoms include weight loss, anxiety, tremors, rapid heart rate, sweating, heat intolerance, and insomnia. Graves’ disease is a common cause. Managing hyperthyroidism often includes antithyroid medications, radioactive iodine therapy, or surgery to reduce thyroid hormone production and bring TSH levels back to a normal range.

TSH and thyroid function testing: TSH is a sensitive marker for evaluating thyroid health and is often tested alongside Free T4 and Free T3 for a complete thyroid profile. High TSH typically indicates hypothyroidism, while low TSH signals hyperthyroidism. In cases of abnormal TSH, additional tests, such as thyroid antibodies, may help diagnose underlying autoimmune thyroid conditions.