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What Is Secondary Hypothyroidism?

Learn if you have secondary hypothyroidism and how it can affect your pituitary gland and TSH levels.
What Is Secondary Hypothyroidism?
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Fatigue, a slow heart rate, brain fog, and weight gain – they’re all signs of hypothyroidism, an underactive thyroid. Hypothyroidism is the most common condition affecting the thyroid, the master gland of metabolism. With hypothyroidism, the gland cannot produce sufficient amounts of thyroid hormone, leaving hypothyroid patients with a thyroid hormone deficiency. Because thyroid hormone is crucial to the body’s energy and metabolism, a lack of thyroid hormone causes body systems to slow down. The effects of thyroid hormone are seen throughout the body – on the brain, the heart, digestion, and the nervous system, among others. When thyroid hormone levels are low, all of these body functions can be slowed. 

The common symptoms of hypothyroidism include:

  • Fatigue
  • Weight gain
  • Depression and anxiety
  • Joint pain
  • Irregular heartbeat
  • Malaise
  • Constipation
  • Dry skin
  • Thinning hair and hair loss 
  • Brittle fingernails
  • Cold intolerance
  • Irregular menstrual periods

Several causes of hypothyroidism are ranked based on where the problem originates in the body. Because thyroid hormone levels are controlled by endocrine glands in the brain, it may be compromised by other malfunctioning organs that tell it what to do.


Primary Hypothyroidism

Primary hypothyroidism is when the thyroid gland cannot make enough thyroid hormone to support the body’s needs. 

In addition to congenital hypothyroidism – a situation where a baby’s thyroid is not producing enough hormone – there are several causes of primary hypothyroidism.


Iodine Deficiency

The first and most common cause of hypothyroidism outside the U.S. is a lack of dietary iodine. The thyroid needs iodine to make thyroid hormones. When iodine is lacking and thyroid hormone cannot be produced, feedback systems from the brain tell the thyroid it needs to produce even more hormone. This can cause a proliferation of thyroid cells and the development of goiter, an enlarged thyroid. In the U.S., the advent of iodized salt in the 1920s significantly reduced levels of iodine deficiency. Still, globally, a lack of iodine currently remains a significant risk for underactive thyroid and goiter, as well as severe disabilities in babies born to women with severe hypothyroidism and iodine deficiency during pregnancy. 

Conversely, too much iodine may also cause hypothyroidism, specifically Hashimoto’s thyroiditis - an autoimmune condition that leads to hypothyroidism. Excess iodine consumption can lead to an overabundance of hydrogen peroxide, which damages thyroid cells via oxidative stress. Cellular death can occur as iodine levels rise.

All this to say, the right amount of iodine is necessary to support healthy cellular function and thyroid hormone production.


Hashimoto’s is the most common cause of hypothyroidism in developed countries where iodine is prevalent. In this autoimmune condition, the immune system flags thyroid cells as foreign and begins to attack them. With time, this leads to chronic inflammation, eventual thyroid gland failure, and an inability to produce thyroid hormones.

Secondary Hypothyroidism

In secondary hypothyroidism, the thyroid gland itself is normal and healthy, but there is a problem with the pituitary gland. The pituitary gland is located in the brain. It is often considered the endocrine system’s “master gland” because – in addition to producing pituitary hormone – it controls most of the other hormone-releasing glands in the body.

Usually, secondary hypothyroidism occurs because the pituitary gland does not release thyroid-stimulating hormone (TSH), also called thyrotropin. TSH is the pituitary’s messenger hormone, communicating to the thyroid gland to ramp up or slow down thyroid hormone production. Several factors can cause secondary hypothyroidism, but a pituitary tumor is the most common cause. Usually, pituitary tumors are benign, but depending on where they are located in the gland, they may produce more or less hormone that targets specific endocrine glands.

Sometimes, secondary hypothyroidism also encompasses problems with the hypothalamus, the endocrine gland that controls the pituitary. However, hypothalamic problems that lead to hypothyroidism are sometimes referred to as tertiary hypothyroidism.

Risk factors for secondary hypothyroidism include:

  • Being over the age of 50
  • Being female
  • Having a positive family history of pituitary or thyroid dysfunction


Diagnosing secondary hypothyroidism

Most cases of hypothyroidism are due to primary causes like Hashimoto’s or iodine deficiency. However, you need to know exactly what is causing your thyroid to be underactive to treat it effectively.

If you suspect something is amiss with your thyroid, you should have a complete thyroid function test that checks the following levels in your blood:

  • TSH: Thyroid stimulating hormone, secreted by the pituitary, tells the thyroid when to produce more T4.
  • Free T4: Thyroxine, the inactive form of thyroid hormone, is made and secreted by the thyroid gland
  • Free T3: Triiodothyronine, the active form of thyroid hormone, is usable by cells.
  • TPO: Thyroid peroxidase antibodies, which are evidence of autoimmune disease.  

In primary hypothyroidism, the TSH level is high because the pituitary constantly receives feedback that more thyroid hormone must be produced. As a result, the pituitary releases more TSH.

In secondary hypothyroidism, thyroid tests are likely to show a low TSH level – usually associated with hyperthyroidism – and low T4 and T3 levels that are consistent with hypothyroidism. 

Your doctor may perform additional lab tests to see if other pituitary hormones are abnormal and recommend MRI imaging to look for a pituitary tumor.

Treating secondary hypothyroidism

After a diagnosis of hypothyroidism, the primary course of treatment is a medication known as thyroid hormone replacement therapy. Your doctor will put you on a dose that will resolve the hypothyroidism and keep your thyroid levels optimal. If other organs, such as your adrenals, are affected, you may need additional treatment to keep your endocrine glands functioning appropriately.

In addition to treatment for hypothyroidism, supportive lifestyle measures can make living with secondary hypothyroidism easier. For example, a high-fiber diet can help relieve constipation, while exercise can help keep your weight in check.

A note from Paloma

If you suspect something is off with your thyroid, make sure you have a complete thyroid panel to test for hypothyroidism. One convenient way to get your thyroid tested is with the Paloma Complete Thyroid Blood Test kit. The affordable Paloma at-home thyroid test kit measures Thyroid Stimulating Hormone (TSH), Free Thyroxine (Free T4), Free Triiodothyronine (Free T3), and Thyroid Peroxidase Antibodies (TPO), with an option to add on tests for Reverse T3 (RT3) and vitamin D.  Hypothyroidism care doesn’t have to be complicated. Consider working with one of Paloma’s team of top thyroid doctors.  You’ll work with a practitioner with expertise in hypothyroidism diagnosis and treatment and high-quality hypothyroidism diagnosis and treatment – all conveniently from the comfort of your home.


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Julia Walker, RN, BSN

Clinical Nurse

Julia Walker, RN, BSN, is a clinical nurse specializing in helping patients with thyroid disorders. She holds a Bachelor of Science in Nursing from Regis University in Denver and a Bachelor of Arts in the History of Medicine from the University of Colorado-Boulder. She believes managing chronic illnesses requires a balance of medical interventions and lifestyle adjustments. Her background includes caring for patients in women’s health, critical care, pediatrics, allergy, and immunology.

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