Reviewed on 11/10/2022

Things to Know about Hypothyroidism

The thyroid gland is located in the neck.
The thyroid gland is located in the neck.

What is the medical definition of hypothyroidism?

  • Hypothyroidism is a condition in which the thyroid gland does not produce enough thyroid hormone.
  • Thyroid hormones affect the metabolic processes of the body.
  • Hypothyroidism can be caused by conditions of the thyroid as well as other diseases which may indirectly affect the thyroid.
  • Women are affected by hypothyroidism more than men, especially as they age.
  • The incidence of hypothyroidism increases significantly in the elderly.

How do I know if I have an underactive thyroid?

  • Low thyroid hormone levels decrease metabolism (how energy is used) and increase the risk of other health issues such as heart disease and problems with pregnancy.

What Causes Adult Hypothyroidism?

6 Common causes of hypothyroidism in adults include:

Autoimmune Thyroiditis (Hashimoto's Thyroiditis)

This inherited condition is the most common cause of hypothyroidism in adults. Hashimoto's thyroiditis is an autoimmune disease in which the body's immune system, or natural defense system, attacks its own thyroid gland. This causes thyroid enlargement or goiter and progressive thyroid destruction.

Subacute Thyroiditis (inflammation of the thyroid gland after a viral disease or after pregnancy)

In this condition, there is usually a phase of hyperthyroidism (a condition in which the thyroid produces an excess of thyroid hormones) followed by a hypothyroid phase. Eventually, thyroid function returns to normal. The three types of subacute thyroiditis are:

  1. subacute granulomatous thyroiditis also referred to as painful thyroiditis;
  2. subacute painless thyroiditis, which is silent and also referred to as lymphocytic thyroiditis; and
  3. postpartum thyroiditis.

Previous Thyroid Therapy

Surgical removal of the thyroid gland may be performed to treat thyroid nodules, hyperthyroidism, or other conditions. Treatment of hyperthyroidism with radioactive iodine also results in the destruction of thyroid tissue and may lead to hypothyroidism.

Drug-induced Hypothyroidism

Taking some prescription medications may alter thyroid function. These include lithium (Eskalith, Lithobid) and amiodarone (Cordarone).

Pituitary and Hypothalamic disease

Both the hypothalamus and pituitary gland are involved in the signaling pathways that control the function of the thyroid gland. Diseases of the hypothalamus and pituitary gland can, therefore, affect the amount of thyroid hormone made and secreted by the thyroid gland. Hypothyroidism due to pituitary disease is termed "secondary hypothyroidism," while hypothyroidism due to hypothalamic disease is termed "tertiary hypothyroidism."

Iodine deficiency

Iodine deficiency does not occur in the U.S. Only severe iodine deficiency will cause low thyroid hormone levels. This condition may occur in mountainous areas of poor, less industrialized nations. Mild to moderate iodine deficiency is common in many European countries.

18 Common Symptoms of Hypothyroidism

Symptoms and signs of hypothyroidism and their degree of severity may vary and depend on the duration and extent of thyroid hormone deficiency. Individuals with hypothyroidism may experience:

  • Fatigue
  • Weakness
  • Intolerance to cold
  • Muscle aching and cramps
  • Constipation
  • Weight gain or difficulty losing weight
  • Poor appetite
  • Goiter (enlarged thyroid gland)
  • Dry, rough skin
  • Coarse hair or hair loss
  • Eye and face swelling
  • Deeper and/or hoarse voice
  • Enlarged tongue
  • Irregular or heavy menstrual periods
  • Depression
  • Memory loss
  • Slowed thinking and mental activity
  • Increased blood cholesterol levels

How to Test for Hypothyroidism

The history and physical examination generally reveal the characteristic symptoms and physical signs that suggest hypothyroidism; however, laboratory evaluation is necessary to establish the diagnosis and cause of hypothyroidism.

Diagnostic lab tests include measuring the blood levels of:

  • Thyroid-stimulating hormone (TSH). This hormone will be elevated in cases of hypothyroidism. The TSH assay is the most sensitive test for the diagnosis of hypothyroidism.
  • Free T4 (free thyroxine) and T3 thyroid hormones. Thyroid hormone levels in the blood will be low, but with mild, or "subclinical" hypothyroidism, the thyroid hormone levels can be in the low normal range.
  • Thyroid autoantibodies (anti-thyroid peroxidase and antithyroglobulin autoantibodies). The presence of these antibodies indicates autoimmune thyroiditis (Hashimoto's disease) as the underlying cause of hypothyroidism.

How to Treat Hypothyroidism

Treatment of hypothyroidism requires life-long therapy (with the exception of certain conditions). 

Here are medications to treat hypothyroidism.

Levothyroxine (L-thyroxine)

The majority of people with hypothyroidism are treated with one of the synthetic forms of the T4 thyroid hormone (Levoxyl, Synthroid). This is a more stable form of thyroid hormone and requires once-a-day dosing, whereas preparations containing T3 (the most active thyroid hormone) are much shorter-acting and need to be taken multiple times a day. Synthetic T4 is readily and steadily converted to T3 naturally in the bloodstream in the great majority of people, and this conversion is appropriately regulated by the body's tissues. A brand name preparation of L-thyroxine is recommended over generic preparations, and individuals should use the same brand of levothyroxine throughout treatment.

Synthroid for Hypothyroidism

Figure 1: The "rainbow" of L-thyroxine dose pills available. For patients taking L-thyroxine, blood levels of TSH should be checked every four to six weeks (when newly started or a dosage or brand change takes place), in order to see if an L-thyroxine dose change is necessary

Other thyroid hormone replacements are available but are not often recommended for replacement therapy. These include:

  • desiccated thyroid hormone,
  • T3 (triiodothyronine), and
  • combinations of thyroid hormones T3 and T4.

Hypothyroidism Follow-Up

Follow-up care for hypothyroidism is important. The optimal adjustment of thyroid hormone dose and patient compliance are critical because the body is sensitive to even small changes in thyroid hormone levels.

After changing the brand or dosage of L-thyroxine, TSH and free T4 levels should be measured within six to eight weeks. Someone who is taking a stable dose of l-thyroxine with previously normal TSH levels should have the TSH checked every 6-12 months.

Related Health Complications of Hypothyroidism

Hypothyroidism may contribute to the development of heart disease. Blood cholesterol levels may be increased, with some studies suggesting an increase in heart disease and heart attacks as a result. L-thyroxine treatment of hypothyroid patients is of value and lowers blood lipid levels by 10%-40%. Moreover, the efficiency of the heart's ability to contract may be reduced with hypothyroidism. Again, treatment with L-thyroxine treatment can reverse these changes.

Thyroid hormone requirements increase during pregnancy. For example, up to half of the pregnant women with Hashimoto's thyroiditis require an increase in L-thyroxine dose during the first 20 weeks of pregnancy. A majority of pregnant women who have a history of thyroid surgery or treatment with radioactive iodine require an increase in the L-thyroxine dose.

Management of hypothyroidism in pregnancy is critical because untreated hypothyroidism can cause complications in pregnancy. Hypothyroid mothers have an increased risk for high blood pressure, low blood counts, and miscarriages, and babies born to hypothyroid mothers are at risk for a lower IQ.

Screening for hypothyroidism during pregnancy with a TSH level test is recommended in women with a previous history of thyroid dysfunction, a family history of thyroid disease and/or another autoimmune disease, or a history of recurrent miscarriages.

Questions to Ask Your Doctor About Thyroid Disease

Note: We recommend you use this page as a reference for your consultation with your doctor.

1. Are my symptoms consistent with thyroid disease?

2. Could my symptoms be the result of anything else?

3. What investigations do I need to have performed to make a diagnosis?

4. How safe are these investigative tests and procedures?

5. What exactly is a thyroid biopsy, and how is it performed?

6. If I do have thyroid disease, how do we treat it?

7. If I need medications, what are the potential side effects?

8. If I need medication, is there any special way I should be taking it?

9. Do we need to monitor blood levels for thyroid function? If so, how often?

10. Is there a possibility my thyroid disease will require surgery?

eMedicineHealth Reminder: Establishing an accurate diagnosis is key to proper treatments. You are the most important person in this process by accurately describing to your doctor the character, location, duration, and time of onset of your symptoms. You should also inform your doctor about vitamins, herbs, and medications you are taking.

Reviewed on 11/10/2022
Medically reviewed by John A. Seibel, MD; Board Certified Internal Medicine with a subspecialty in Endocrinology & Metabolism

REFERENCE: MedscapeReference.com. Hypothyroidism.