The type of hypothyroidism a person has depends on whether it develops due to faults in the thyroid itself or other glands that instruct the thyroid to make hormones.
The thyroid is a small gland in the neck that produces hormones linked to various systems and functions across the body. However, it relies on communication from glands in the brain, such as the pituitary gland or hypothalamus, to send their own hormones that trigger production.
These form part of the hypothalamic-pituitary-thyroid axis that plays an important role in hormone production. The hormones that play a role in normal thyroid function include:
- triiodothyronine (T3) and thyroxine (T4), which the thyroid gland produces
- thyroid-stimulating hormone (TSH), which the pituitary gland produces to trigger T3 and T4 production in the thyroid
- thyrotropin-releasing hormone (TRH), which stimulates TSH production in the hypothalamus
Issues with the production of TSH or TRH may mean that secondary or tertiary hypothyroidism has developed. These occur due to different causes.
This article outlines the different types of hypothyroidism. It also explains symptoms and treatment for the condition. Finally, it discusses some conditions that may be related.
Hypothyroidism occurs when the thyroid gland does not produce enough hormones.
Primary hypothyroidism
Several causes can directly affect thyroid hormone production,
- the autoimmune condition Hashimoto’s thyroiditis
- inflammation of the thyroid, known as thyroiditis
- congenital hypothyroidism
- removal of some or all of the thyroid during surgery
- damage to the thyroid during radiation treatment with iodine
- medication side effects
Healthcare professionals diagnose primary hypothyroidism through a range of blood tests that detect levels of thyroid hormones in the blood. People with primary hypothyroidism generally have low T3 and T4 but high levels of TSH, as the pituitary gland normally reduces TSH levels when it detects that the thyroid has produced enough T3 and T4.
Secondary hypothyroidism occurs when thyroid levels are too low
It is one of two types of central hypothyroidism, which healthcare professionals often use to describe either secondary or tertiary hypothyroidism.
A 2017 review estimates that
The causes of secondary hypothyroidism include:
- pituitary tumors
- Sheehan syndrome, a rare condition involving the necrosis (death) of pituitary tissue
following issues during childbirth - lymphocytic hypophysitis, a
rare condition in which the immune system attacks healthy tissue in the pituitary gland - radiation therapy damage in the brain
- medication side effects, including from:
- dopamine
- prednisolone
- opioids
Blood tests allow healthcare professionals to diagnose secondary hypothyroidism. TSH, T3, and T4 will generally be low. However, secondary hypothyroidism does not typically reduce levels of TRH.
Tertiary hypothyroidism occurs due to hypothalamus damage, which leads to low levels of TRH. In turn, this affects how much TSH the pituitary produces, which then reduces levels of thyroid hormones.
Causes of tertiary hypothyroidism may include:
- tumors that push on the hypothalamus
- TRH resistance, where TRH is high
- TRH deficiency
Blood tests for tertiary hypothyroidism will generally show low levels of T3, T4, TSH, and TRH.
Each type of hypothyroidism can lead to a range of slow-developing, subtle symptoms that may be mistaken for the effects of other conditions.
These symptoms can
- extreme tiredness
- unintended or unexpected weight gain
- reduced cold tolerance
- painful joints and muscles
- dry skin
- thinning, dry hair
- changes to fertility or the menstrual cycle
- a slow heart rate
- depression
Without treatment, hypothyroidism can lead to high cholesterol and heart problems. Severe hypothyroidism can also cause myxedema coma, which can be life threatening and requires emergency treatment.
Treating all types of hypothyroidism means that a person
They can take a medication called levothyroxine. This is a lab-made form of T4 that fulfills the roles that natural T4 would have carried out in people with healthy thyroid levels.
Healthcare professionals will monitor doses and levels yearly to measure the right amount for resolving symptoms.
Some conditions are not separate types of hypothyroidism, but they
Hashimoto’s thyroiditis
Hashimoto’s thyroiditis is the most common cause of hypothyroidism. The immune system mistakenly attacks the thyroid gland, which then develops inflammation. After this, the thyroid can no longer produce enough hormones.
Learn more about Hashimoto’s thyroiditis.
Thyroiditis
Thyroiditis is inflammation in the thyroid that causes a leakage of hormones from the gland. This causes an initial spike of thyroid hormones, leading to thyrotoxicosis, or extremely high thyroid hormones. Over time, the thyroid becomes less active, sometimes contributing to permanent hypothyroidism.
This leads to swelling and can be painful for some people. Thyroiditis may also develop in some people shortly after giving birth.
Congenital hypothyroidism
If an infant has an underdeveloped or faulty thyroid at birth, they can develop congenital hypothyroidism. Without treatment, this may lead to intellectual disability and low growth. However, early treatment is effective at managing these developmental issues. In the United States, every infant receives thyroid testing.
Learn about common thyroid disorders.
The following are answers to some questions people frequently ask about types of hypothyroidism.
How can you tell the difference between hypothyroidism and Hashimoto’s?
Hypothyroidism refers to low thyroid hormone levels. However, it can result from many different health problems and conditions. The
How do I know what type of hypothyroidism I have?
Only blood tests can identify a particular type of hypothyroidism:
- Primary hypothyroidism causes low T3 and T4 but high TSH
- Secondary hypothyroidism causes low T3, T4, and TSH
- Tertiary hypothyroidism causes low T3, T4, TSH, and TRH
The type of hypothyroidism a person has depends on which part of the hypothalamic-pituitary-thyroid axis is responsible for low hormone levels.
Primary hypothyroidism is overwhelmingly the most common type, which often occurs due to Hashimoto’s thyroiditis, congenital hypothyroidism, or thyroiditis.
Central hypothyroidism, which is much rarer and includes secondary and tertiary hypothyroidism, affects the pituitary gland and hypothalamus. The pituitary may produce less thyroid-stimulating hormone (TSH), or the hypothalamus less thyrotropin-releasing hormone (TRH).
Symptoms and treatments are similar for all types. If an individual is experiencing unexpected or unexplained weight gain, extreme tiredness, and painful joints or muscles, they should speak with a healthcare professional.