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Several different organs are crucial for thyroid function. Indeed, the thyroid gland does not work solely on its own but instead is part of a more extensive system responsible for regulating metabolism and homeostasis. The pituitary gland is one of the critical organs that helps the thyroid do its job.
Ahead, we explore everything about the pituitary gland, including its role in thyroid function.
The pituitary gland is a small, pea-shaped organ housed deep inside the brain. It is an endocrine gland, which means it produces hormones. Sometimes, the pituitary gland is called the "master gland" because it produces its own hormones and tells other endocrine glands how to function.
The pituitary gland resides just below the hypothalamus, which gives directions to the autonomic nervous system about blood pressure, respiratory rate, digestion, and heart rate. The hypothalamus tells the pituitary gland what to do via its own hormones so that the pituitary can control the rest of the endocrine glands in the body.
Anatomically, the pituitary gland is divided into two sections: the anterior lobe and the posterior lobe. There is a connection of blood vessels and nerves (sometimes referred to as a stalk) that unites the hypothalamus and pituitary. Hormones (or chemical messengers) are transferred from the hypothalamus to the pituitary, so it knows how to direct the actions of other hormone-secreting glands.
The pituitary gland makes and releases several hormones specific to other endocrine glands. These hormones tell other endocrine glands how many hormones they should be making and when they should release them.
The primary hormones made and secreted by the pituitary include:
- Adrenocorticotrophic hormone (ACTH): Tells the adrenal glands how much cortisol (or stress hormone) to make.
- Growth hormone (GH): stimulates bone and muscle production and helps with fat distribution.
- Thyroid-stimulating hormone (TSH): tells the thyroid how much thyroid hormone (T4 and T3) to make to support metabolism and energy.
- Antidiuretic hormone (ADH): also called vasopressin, this hormone regulates the balance of water and sodium in the body.
- Follicle-stimulating hormone (FSH): supports sperm production, egg maturation in the ovaries, and estrogen.
- Luteinizing hormone (LH): Makes testosterone in men and stimulates ovulation.
- Prolactin (PRL): primarily stimulates milk production after childbirth and influences male and female hormones that can affect sexual function, fertility, and menstruation.
- Oxytocin: responsible for labor progression, breast milk production and flow, and socialization between a mother and baby.
The pituitary gland is a part of several axes that are part of feedback loops to keep systems in check. The hypothalamus-pituitary-thyroid axis (which will now be referred to as the HPTA) is perhaps the most important in controlling metabolism and energy.
The primary role of the HPTA is to maintain normal thyroid hormone levels circulating in the bloodstream.
Normal levels of thyroid hormone regulate:
- Brain development
- Bone and muscle formation
- Liver function
- Food intake and digestion
- Energy expenditure
- Reproductive function
- Cardiovascular function
The pituitary releases TSH to tell the thyroid how much thyroid hormone to make. The hormones produced by the thyroid are thyroxine (T4) and triiodothyronine (T3). About 80% of thyroid hormone in the blood is T4, and 20% is T3 (although T3 is much more potent and is the active form of this hormone). Tissues like the liver and kidneys convert T4 into T3, which becomes activated and usable by cells.
When thyroid hormone levels are high enough or at the right level, a feedback system tells the hypothalamus and thus the pituitary that it can decrease the amount of TSH because levels are high enough. This system is called a negative feedback loop.
Thyroid diseases and the HPTA
Thyroid diseases can throw off this feedback loop. For example, when a thyroid gland is hypoactive, it does not release enough T4 to support the body's metabolic needs. Several factors can cause hypothyroidism, but the most common cause in the United States is Hashimoto's disease. In this condition, the immune system mistakenly attacks healthy thyroid cells, causing chronic inflammation and the inability of these cells to make T4.
When T4 and T3 are low, the pituitary receives messages that there isn't enough thyroid hormone. It subsequently secretes more TSH to tell the thyroid to make more T4. For this reason, we usually take blood samples to measure TSH levels to get an idea of how the thyroid is functioning. While TSH levels are insightful, they do not provide the whole picture about the HPTA. Thus, it is important to check a complete thyroid blood panel that includes TSH, free T4, free T3, and TPO antibodies.
Because the pituitary is so pivotal in controlling other endocrine glands in the body, it goes without saying that problems with the pituitary can wreak havoc on your whole system.
The most common pituitary disorders are pituitary tumors or adenoma.
Fortunately, 99% of pituitary tumors are benign and considered non-functional, meaning they do not produce excessive amounts of hormones. About 10% of the population has small, non-functional pituitary adenomas. They are usually found incidentally during brain scans for other concerns.
However, some pituitary adenomas will be functional and produce excess hormones, leading to conditions like acromegaly and Cushing's syndrome. Specifically, to the thyroid, a TSH-secreting pituitary adenoma is also benign (and rare). Still, it can lead to excess TSH production, causing thyroid gland enlargement (goiter) and overactive thyroid (hyperthyroidism).
If you feel your thyroid function may be off, the best place to start is with a thyroid blood test. The markers assessed in Paloma Health's at-home thyroid test kit can tell your health care provider a lot about how your HPTA axis functions. Thyroid test results can also give insight into whether or not an autoimmune condition may be behind your thyroid-related symptoms.