Metabolism refers to the chemical reactions inside cells that convert food into energy. A metabolic disorder is any disorder that affects metabolism. Hypothyroidism is a metabolic disorder because it involves the underproduction of hormones that play a significant role in metabolism.
Hypothyroidism happens when the thyroid gland does not produce enough of the thyroid hormones triiodothyronine (T3) and thyroxine (T4). These hormones play a role in many metabolic processes throughout the body. As such, people with hypothyroidism report a wide range of nonspecific symptoms, from unintentional weight loss to dry skin.
This article describes metabolic disorders and the different types of hypothyroidism in more detail. It also outlines the causes, symptoms, diagnosis, and treatment of hypothyroidism, as well as the outlook for people with this condition.
Metabolism refers to chemical reactions that convert food into energy. Metabolic disorders involve abnormal chemical reactions that disturb the body’s metabolism. A metabolic disorder is any disease that affects how the body produces or processes a major building block, such as proteins or nucleic acids.
Metabolic disorders can happen when organs become diseased or stop functioning normally. An example is type 2 diabetes, which involves impaired pancreatic functioning.
Some metabolic disorders can happen as a result of mutations in certain genes. Experts have identified hundreds of these diseases, most of which are rare. Examples include:
- dihydropyrimidine dehydrogenase deficiency
- Gilbert syndrome
- mannose-binding lectin protein deficiency
- monogenic diabetes
- peroxisomal disorders
Hypothyroidism is a metabolic disorder because it involves the underproduction of hormones that play a role in metabolism.
The thyroid is a butterfly-shaped gland in the front of the neck. It produces the thyroid hormones T3 and T4. These hormones affect almost every physiological process in the body, including metabolism.
People living with hypothyroidism do not produce enough thyroid hormones, which causes many of the body’s functions to slow down. Extreme hypothyroidism can cause bodily functions to slow to a life threatening extent.
Low levels of thyroid hormones in the blood characterize hypothyroidism, which doctors classify into two categories: primary and secondary.
In people with primary hypothyroidism (PH), the thyroid gland is unable to produce thyroid hormones. Iodine deficiency is the most common cause of PH.
In those with secondary hypothyroidism (SH), the thyroid gland functions normally. People with SH may have a problem with their pituitary gland or hypothalamus, both of which are regions of the brain.
Globally, the most common cause of hypothyroidism is a lack of iodine in the diet. However, in the United States, an autoimmune disease called Hashimoto disease is most likely responsible. Other possible causes include:
- medications, such as:
- a new combination therapy using the anticancer drugs anti-cytotoxic T-lymphocyte-associated antigen (CTLA-4) and anti-programmed cell death ligand 1 (PD-L1/PD-1)
- thyroid radioactive iodine therapy
- thyroid releasing hormone (TRH) resistance
- TRH deficiency
- thyroid surgery
- radiation therapy of the head, neck, or brain
- disorders of the pituitary gland or hypothalamus
- Sheehan syndrome
- lymphocytic hypophysitis
The symptoms of hypothyroidism may vary from person to person, and some individuals may not have any noticeable symptoms. When symptoms do appear, they may be mild and nonspecific. Typical symptoms include:
- intolerance to cold temperatures
- skin changes, such as dryness
- decreased sweating
- hair loss
- weight gain
- difficulty sleeping
- muscle cramps
- changes to the menstrual cycle
- milky nipple discharge
Doctors cannot rely on symptoms alone to diagnose hypothyroidism. They will make a diagnosis by:
- taking a full medical history and family history
- conducting a physical examination
- requesting medical imaging of the thyroid gland
- requesting blood tests to check the levels of thyroid hormones
Doctors typically treat hypothyroidism with the prescription medication levothyroxine (Synthroid), which contains synthetic thyroid hormone. Once a person starts taking the medication, their hormone levels quickly rise.
Doctors will calculate the starting dose of levothyroxine based on a person’s weight. Older adults and people with the heart condition atrial fibrillation may need lower dosages.
A doctor will check the person’s thyroid hormone levels at about 4–8 weeks to ensure that the medication is working. If the thyroid hormone levels are still too low or have risen too much, the doctor may need to adjust the dosage. After making an adjustment, they will repeat the blood tests until the person’s hormone levels are within the healthy range.
People with untreated hypothyroidism have a high risk of complications, such as heart failure, coma, and death. The leading cause of death with untreated hypothyroidism is heart failure. People with extreme hypothyroidism can go into a severe type of coma called myxedema coma, which has a very high rate of death.
Most people living with hypothyroidism see their symptoms lessen within a few weeks or months of starting levothyroxine.
If the dosage of levothyroxine is too high, the drug can cause side effects. These side effects are the same as the symptoms of hyperthyroidism, which is the medical term for an overactive thyroid.
The side effects of levothyroxine may include:
- intolerance to heat
- skin rash
- muscle spasms or shaking
- muscle weakness
- difficulty sleeping
- increased hunger
- unexplained weight loss
- changes to the menstrual cycle
Serious side effects of levothyroxine include:
- shortness of breath
- abnormal heartbeat
- heart attack
With appropriate monitoring and blood tests, doctors can adjust the prescribed dosage to help people manage side effects.
Metabolism refers to all the chemical reactions that take place inside the body’s cells when converting food into energy. Any disease that affects how the body breaks down, processes, or produces key substances involved in metabolism is a metabolic disease.
Hypothyroidism is the medical term for abnormally low levels of thyroid hormones in the blood. It is one of many metabolic diseases. In hypothyroidism, low levels of thyroid hormone cause the body’s metabolism to slow down. This can affect multiple bodily functions and cause a wide range of symptoms.
Doctors treat hypothyroidism with medications that contain a synthetic version of thyroid hormone. People typically begin to feel better soon after starting treatment. A person will initially require blood tests to ensure that the dosage is correct. Dosages that are too high may result in side effects, some of which could be severe. Anyone who experiences side effects while taking the medication should notify a doctor as soon as possible.