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Several different hormone-producing glands participate in supporting thyroid health. When one of those organs is not functioning optimally, it can throw the whole system off. And when the hypothalamus is the culprit, as is the case in tertiary hypothyroidism (also known as central hypothyroidism), it can make it challenging to treat thyroid dysfunction.
There are three main organs involved in thyroid hormone production. The hypothalamus is the organ that initiates the feedback loop, followed by the pituitary gland, and then the thyroid.
The hypothalamus is the organ that controls all of the endocrine system. It lives in the middle of the brain and secretes hormones to the pituitary (another endocrine gland in the brain), telling it what to do.
Because it oversees the entire endocrine system, it truly affects every organ in the body. Among its many roles, the hypothalamus connects the endocrine system to the nervous system and controls homeostasis. One of its critical roles with the endocrine system is its ability to influence and communicate with the autonomic nervous system to maintain a steady internal state.
The hypothalamus controls homeostasis by regulating:
- Heart rate
- Blood pressure
- Body temperature
- Electrolyte balance (and therefore thirst)
- Sleep-wake cycles
Many of these functions come from the signals (or hormones) it releases to the pituitary, which tells the rest of the endocrine system what to do.
Hormones produced by the hypothalamus include:
- Antidiruetic hormone (ADH)
- Corticotropin-releasing hormone (CRH)
- Gonadotropin-releasing hormone (GnRH)
- Growth hormone-releasing hormone (GHRH)
- Growth hormone-inhibiting hormone (GHIH)
- Prolactin-releasing hormone
- Prolactin-inhibiting hormone
- Thyrotropin-releasing hormone (TRH)
The pituitary gland is a pea-sized endocrine gland that lies below the hypothalamus in the brain. A stalk of nerves and blood vessels connects these two organs so they can support the rest of the endocrine system in the body.
While the hypothalamus has the upper hand in controlling the pituitary and the rest of the hormone-producing glands in the body, the pituitary is considered the "master gland" because it produces and secretes numerous hormones that reach endocrine glands throughout the body.
Along with hormones to control fertility, growth, and the adrenal glands (to name a few), the pituitary also secretes TSH (thyroid-stimulating hormone). This hormone tells the thyroid how much thyroid hormone to procure and secrete.
The thyroid is the metabolic powerhouse of the body. This butterfly-shaped organ lives at the nape of the neck, and its primary role is to make thyroid hormones that tell cells how to utilize their energy.
Thyroid hormones help regulate numerous processes, including heart rate, blood pressure, digestion, bone reabsorption, the menstrual cycle, and growth and development. Problems with the thyroid gland or the organs that help control it can throw off the homeostatic balance in the body.
There are three different classes of hypothyroidism. Tertiary hypothyroidism (or sometimes called central hypothyroidism) is when something is not right with the hypothalamus, causing the thyroid to underproduce thyroid hormone. There is usually nothing wrong with the thyroid gland itself or even the pituitary.
In tertiary hypothyroidism, the hypothalamus does not release enough thyrotropin-releasing hormone, so the pituitary does not send enough TSH to encourage thyroid hormone (T3 and T4) production.
Tertiary hypothyroidism is rare, but it is often due to a mass or tumor near or on the hypothalamus when it does occur.
Symptoms of tertiary hypothyroidism:
- Muscle pain or weakness
- Joint pain or weakness
- Puffy face, hands
- Difficulty hearing
- Hair loss or thinning
- Dry skin, hair, or nails
- Poor concentration
- Cold intolerance
- Poor memory or brain fog
- Carpal tunnel syndrome
- Weight gain
- Hoarse voice
As you probably can guess, there are other classifications of hypothyroidism: primary and secondary.
Primary hypothyroidism, where the problem lies within the thyroid itself, is the most common cause of an underactive thyroid. Worldwide, most cases of primary hypothyroidism are from a lack of dietary iodine, and this mineral plays a crucial role in the formation of thyroid hormones inside thyroid cells. However, in developed nations where iodized salt is abundant, primary hypothyroidism is usually caused by Hashimoto's disease.
Hashimoto's is an autoimmune condition where the immune system becomes overactive and attacks healthy cells in the thyroid gland. By constantly attacking the thyroid, chronic inflammation can occur, resulting in eventual organ failure.
Less common is hypothyroidism induced by a problem in the pituitary gland. Usually, this type of hypothyroidism is because of a pituitary tumor that causes an under secretion of TSH. Both secondary and tertiary hypothyroidism are also known as central hypothyroidism.
To make a diagnosis of hypothyroidism, you should start with a complete thyroid function panel.
A complete thyroid panel should include:
If your thyroid hormones (T3 and T4) are low but TSH is normal, it indicates something may be wrong with the thyroid gland but not the glands above it in the brain. This may be drug-induced, related to Hashimoto's, or perhaps an iodine deficiency.
If T4, T3, and TSH are all low, it indicates there may be a problem with either the pituitary or hypothalamus. Your doctor may then request an MRI scan of your brain to see if a tumor is present on or near either endocrine gland. In the past, a thyrotropin-releasing hormone test may have been performed, but there is not much evidence to show this test is necessary, and this practice is now outdated.
The goal of treatment in patients with secondary or tertiary hypothyroidism is to restore normal levels of thyroid hormones with thyroid hormone replacement medication.
Typically, thyroid medication dosing is easily adjusted in patients with primary hypothyroidism based on TSH levels. However, in hypothyroid patients with secondary or tertiary hypothyroidism, it's more helpful to look at T4 levels during treatment to adjust medication dosing.
Before starting treatment for tertiary hypothyroidism, your healthcare provider should rule out adrenal insufficiency, in which case a different treatment plan may be beneficial.
When choosing thyroid medication with your doctor, remember that there is no one-size-fits-all treatment.