What is the function of the thyroid gland?
Thyroid gland is a gland located in front of the neck. This produces thyroid hormones (thyroxine, T4 and tri-iodothyronine, T3) which enter the blood circulation and act on several tissues of the body to maintain optimal body function.
What is the main regulator of thyroid hormone production from thyroid gland?
Thyroid hormone production by thyroid gland is regulated by the master gland 'Pituitary' through another hormone 'thyroid stimulating hormone, TSH'. If the thyroid hormone (T4) production by thyroid gland is reduced, the master gland senses it and signals thyroid gland to produce more thyroid hormones by increasing TSH level in the blood.
Why is TSH often elevated but T4 is normal in hypothyroidism patients?
A slight decrease in production of thyroid hormone is sufficient for the master gland 'Pituitary' to recognize the reducing ability of thyroid gland to produce thyroid hormones. During the initial phase of hypothyroidism, increased TSH drives thyroid gland to work harder to maintain normal T4 level. Hence, most often hypothyroidism patients present with high TSH and normal T4. However, in more advanced damage to thyroid gland, it fails to produce adequate amount of T4 despite elevated TSH. This leads to the classical hormonal profile of hypothyroidism, elevated TSH and low T4.
Note: T4 levels may be normal but TSH is always elevated in patients with primary hypothyroidism.