Thyroid Stimulating Hormone (TSH)
Thyroid Stimulating Hormone (TSH) is a hormone produced by the pituitary gland, located at the base of the brain. It plays a crucial role in regulating thyroid function by stimulating the thyroid gland to produce thyroid hormones, primarily thyroxine (T4) and to a lesser extent, triiodothyronine (T3). TSH levels are used to evaluate thyroid function in several ways:
- Primary screening for thyroid disorders: TSH is often used as the initial test to assess thyroid function. An elevated or suppressed TSH level can indicate thyroid dysfunction, prompting further investigation with additional thyroid function tests.
- Hypothyroidism (underactive thyroid): Elevated levels of TSH are typically seen in primary hypothyroidism, where the thyroid gland fails to produce sufficient thyroid hormones (T4 and T3). High TSH levels indicate that the pituitary gland is producing more TSH in an attempt to stimulate the thyroid gland to produce more thyroid hormones.
- Hyperthyroidism (overactive thyroid): Conversely, suppressed levels of TSH are often observed in primary hyperthyroidism, characterized by excessive production of thyroid hormones by the thyroid gland. Low TSH levels indicate that the pituitary gland is producing less TSH due to feedback inhibition from high levels of thyroid hormones.
- Subclinical thyroid dysfunction: Subclinical hypothyroidism or hyperthyroidism refers to mild thyroid dysfunction with abnormal TSH levels but normal levels of thyroid hormones (T4 and T3). Subclinical hypothyroidism is diagnosed when TSH is elevated with normal T4 levels, while subclinical hyperthyroidism is diagnosed when TSH is suppressed with normal T4 levels.
- Monitoring thyroid replacement therapy: TSH levels are monitored in individuals undergoing treatment for hypothyroidism with thyroid hormone replacement therapy (e.g., levothyroxine). The goal of therapy is to normalize TSH levels within the reference range, indicating adequate thyroid hormone replacement.
- Differentiating primary from secondary thyroid disorders: In cases of thyroid dysfunction, the pattern of TSH secretion can help differentiate primary thyroid disorders (e.g., primary hypothyroidism or hyperthyroidism) from secondary thyroid disorders due to dysfunction of the pituitary gland or hypothalamus.