OVERVIEW

What is primary hypothyroidism?

The pituitary gland and hypothalamus in the brain work together to "direct" the thyroid gland to synthesize and secrete thyroid hormones (including T4 and T3). The pituitary gland accomplishes this "direction" by secreting thyroid-stimulating hormone (TSH), which acts on the thyroid.

Hypothyroidism is a condition caused by various factors that lead to reduced secretion of T4 and T3 by the thyroid or weakened effects of T4 and T3. It can result in symptoms such as fatigue, cold intolerance, constipation, and weight gain.

Depending on the cause, hypothyroidism can be classified into three types: primary hypothyroidism, central hypothyroidism, and thyroid hormone resistance syndrome.

Primary hypothyroidism, abbreviated as primary hypothyroidism, refers to reduced secretion of T4 and T3 due to thyroid gland diseases (such as Hashimoto's thyroiditis or thyroidectomy). In this case, with the help of the hypothalamus, the pituitary gland secretes more TSH in an attempt to increase the "production" of T4 and T3. Therefore, when primary hypothyroidism occurs, blood tests for thyroid function will show elevated TSH and normal or decreased T4 and T3 levels.

The main treatment for primary hypothyroidism is thyroid hormone replacement therapy, commonly using levothyroxine tablets. It usually requires lifelong treatment but has little impact on normal life and work.

Is primary hypothyroidism common?

Studies show that the prevalence of hypothyroidism in China is 17.8%, meaning about 18 out of 100 people have the condition, and most cases are primary hypothyroidism.

What are the types of primary hypothyroidism?

Based on the severity of thyroid dysfunction, it can be divided into subclinical hypothyroidism and clinical hypothyroidism.

• Subclinical hypothyroidism: A milder form, which may present with no symptoms of hypothyroidism and only changes in thyroid function. Thyroid function tests show elevated TSH but normal T4 and T3 levels.

• Clinical hypothyroidism: A more severe form than subclinical hypothyroidism, presenting with both symptoms of hypothyroidism and changes in thyroid function. Thyroid function tests show elevated TSH and decreased T4 and T3 levels.

SYMPTOMS

What are the common manifestations of primary hypothyroidism?

Early-stage patients with mild conditions may not exhibit specific symptoms.

Typical patients may experience cold intolerance, fatigue, swelling sensation in hands and feet, excessive sleepiness, memory impairment, reduced sweating, joint pain, weight gain, and constipation. Women often also experience menstrual irregularities, menorrhagia, or infertility.

During examination, doctors may observe patients with an expressionless face, slow reactions, hoarse voice, hearing loss, pale complexion, facial/eyelid edema, thick lips and enlarged tongue (often with tooth marks), dry, rough, and scaly skin, low skin temperature, edema, yellowish discoloration of palms and soles, sparse and dry hair, and slow pulse. A few patients may also develop lower limb edema.

Can primary hypothyroidism lead to serious consequences?

This condition can cause pericardial effusion and heart failure. Severe cases may progress to coma, which can be life-threatening.

CAUSES

What causes primary hypothyroidism?

The causes of primary hypothyroidism are all related to thyroid diseases, including:

• Autoimmune thyroiditis: such as Hashimoto's thyroiditis, atrophic thyroiditis, etc.;

• Post-thyroidectomy;

• After radioactive ¹³¹I therapy for hyperthyroidism or neck radiotherapy;

• Excessive or deficient iodine intake;

• Drug effects: such as antithyroid drugs (e.g., thioamides), lithium carbonate, phenylbutazone, etc.;

• Thyroid amyloidosis;

• Congenital absence of the thyroid or ectopic thyroid (thyroid tissue located in abnormal sites such as the pharynx, tongue, sternum, trachea, or retrosternal area instead of the neck);

• Genetic abnormalities affecting thyroid hormone synthesis.

Who is more likely to develop primary hypothyroidism?

Women are more susceptible than men. The prevalence increases with age.

DIAGNOSIS

What tests are needed for primary hypothyroidism?

Thyroid function tests, thyroid autoantibodies, thyroid ultrasound or radionuclide scan, complete blood count, lipid profile, etc.

Why are these tests necessary for primary hypothyroidism? What is their purpose?

• Thyroid function tests: Used to diagnose whether it is primary hypothyroidism, assess its severity, and determine if it is clinical or subclinical hypothyroidism. Primary hypothyroidism shows elevated thyroid-stimulating hormone (TSH). In subclinical hypothyroidism, only TSH is elevated, whereas clinical hypothyroidism is also accompanied by decreased T4 and T3 levels.

• Thyroid autoantibodies: Including thyroid peroxidase antibody (TPOAb) and thyroglobulin antibody (TgAb), these help identify the cause and determine if autoimmune thyroiditis is present.

• Thyroid ultrasound or radionuclide scan: Used to assist in diagnosing the underlying cause.

• Complete blood count: Used to determine whether primary hypothyroidism has caused anemia.

• Lipid profile: Used to assess whether primary hypothyroidism has led to dyslipidemia, as hypothyroidism may cause hyperlipidemia.

Are there any precautions for testing primary hypothyroidism?

Thyroid function and autoantibody tests require blood draws but do not require fasting—they can be performed after eating.

Thyroid ultrasound and radionuclide scans also do not require fasting.

Which diseases are easily confused with primary hypothyroidism?

This condition may be confused with central hypothyroidism, adrenal insufficiency, pituitary TSH-secreting adenoma, sick euthyroid syndrome, and thyroid hormone resistance syndrome. Differentiation can be made based on specific symptoms and the aforementioned tests.

TREATMENT

Which department should I visit for primary hypothyroidism?

Endocrinology.

Does primary hypothyroidism always require treatment?

Not necessarily. Some cases of subclinical hypothyroidism do not require treatment. You can directly click on "Subclinical Hypothyroidism" to view related information.

How is primary hypothyroidism treated?

Replace what is missing. Primary hypothyroidism is caused by insufficient thyroid hormone production, so simply supplementing thyroid hormone is sufficient.

Levothyroxine is the main treatment for this condition. Common levothyroxine medications include Euthyrox, L-Thyroxine, and Letrox.

What precautions should be taken when using levothyroxine for primary hypothyroidism?

Levothyroxine only needs to be taken once a day, usually on an empty stomach in the morning, 0.5 to 1 hour before breakfast, as some foods can interfere with its absorption. If taking it in the morning on an empty stomach is not possible, it can also be taken at bedtime, at least 4 hours after dinner.

Levothyroxine should be taken at least 4 hours apart from other medications, as some drugs can affect its absorption and metabolism, such as aluminum hydroxide, calcium carbonate, cholestyramine, sucralfate, ferrous sulfate, and dietary fiber supplements.

Additionally, certain medications can accelerate the clearance of levothyroxine, such as phenobarbital, phenytoin, carbamazepine, rifampin, isoniazid, lovastatin, amiodarone, sertraline, and chloroquine. If taking these medications, the dose of levothyroxine may need to be increased.

In summary, if hypothyroidism patients are taking other medications, they should inform their doctor in advance so the doctor can determine the appropriate dosage and schedule for taking the medications.

Is follow-up testing required after treatment for primary hypothyroidism? How is it done?

Yes.

• In the early stages of treatment, follow-up visits should occur every 1–2 weeks or even sooner. The doctor will evaluate clinical symptoms, heart rate, etc., to determine if the levothyroxine dose is too low or too high. Thyroid function tests should be performed every 4–6 weeks, and the medication dosage should be adjusted based on the results until the treatment target is reached.

• After achieving the treatment target, follow-up testing of the above indicators should be done at least every 6–12 months.

Can primary hypothyroidism be cured?

• Primary hypothyroidism caused by conditions like subacute thyroiditis or painless thyroiditis is usually temporary, and most patients recover on their own.

• Primary hypothyroidism caused by medications can improve after reducing or discontinuing the relevant drugs.

• Primary hypothyroidism caused by other factors is difficult to cure or improve spontaneously and usually requires lifelong levothyroxine treatment. However, treating primary hypothyroidism is not troublesome, and side effects are rare when the dosage is appropriate. Therefore, even if lifelong medication is needed, the impact on daily life is minimal, and there is no need to feel overly burdened.

DIET & LIFESTYLE

What should patients with primary hypothyroidism pay attention to in their diet?

• For hypothyroidism caused by iodine deficiency, iodized salt should be consumed regularly, along with iodine-rich foods such as seaweed, kelp, and shellfish.

• For hypothyroidism caused by autoimmune thyroiditis, excessive intake of iodine-rich foods should be avoided. Iodized salt can still be used, but high-iodine foods like seaweed and kelp should be minimized or avoided.

• Hypothyroidism may lead to hyperlipidemia, so patients should maintain a light, low-fat diet and reduce consumption of fatty meats, animal skin, cream, and fried foods.

Can patients with primary hypothyroidism have normal fertility?

Untreated primary hypothyroidism during pregnancy can adversely affect both the mother and fetus. With active treatment, these risks can be minimized. Patients diagnosed before pregnancy should maintain thyroid-stimulating hormone (TSH) levels between 0.1–2.5 mIU/L before conception.

Is primary hypothyroidism always hereditary?

Primary hypothyroidism itself is not hereditary, but certain underlying thyroid conditions that cause it may be, such as autoimmune thyroiditis or genetic abnormalities in thyroid hormone synthesis.

PREVENTION

Can primary hypothyroidism be prevented?

Some causes of primary hypothyroidism can be prevented, such as avoiding iodine deficiency or excessive iodine intake, and avoiding the use (or overuse) of drugs that inhibit thyroid synthesis.