Secondary hypothyroidism involves decreased activity of the thyroid caused by failure of the pituitary gland.
Causes, incidence, and risk factors
The thyroid gland is an important organ of the endocrine system, located in the front of the neck just below the voicebox. The thyroid secretes the hormones thyroxine (T4), triiodothyronine (T3), and calcitonin, which control body metabolism and regulate calcium balance.
The secretion of T3 and T4 by the thyroid is controlled by an endocrine feedback system involving the pituitary gland and the hypothalamus (structures in the brain). Lowered levels of these thyroid hormones result in increased levels of pituitary and hypothalamic hormones. The reverse is also true - when levels of the thyroid hormones rise, pituitary and hypothalamic hormones fall back. This helps keep levels appropriately balanced.
Since the thyroid gland is regulated by the pituitary gland and the hypothalamus, thyroid disorders may result not only from defects in the thyroid itself but also from the disruption of the control system in these other organs.
Thyroid disorders caused by overproduction of thyroid hormones are called hyperthyroidism, and underproduction of these hormones is known as hypothyroidism.
The cause of secondary hypothyroidism is failure of the pituitary gland to secrete thyroid stimulating hormone (TSH). This is usually caused by a tumor in the region of the pituitary. Rarely the cause is an infiltration of the pituitary by inflammatory cells from the immune system or foreign substances (such as iron in hemochromotosis).
Hypothyroidism may cause a variety of symptoms and can affect all body functions. The body’s normal rate of functioning slows, causing mental and physical sluggishness. Symptoms vary from mild to severe. The most severe form is called myxedema, which is a medical emergency and can lead to coma and death.
Risk factors for secondary hypothyroidism include being over 50 years old, being female, and having a history of pituitary or hypothalamic dysfunction.
- Cold intolerance
- Weight gain
- Joint or muscle pain
- Brittle fingernails
- Coarseness, thinning of hair
- Slow speech
- Dry, flaky skin
- Thickening of the skin
- Puffy face, hands, and feet
- Decreased hearing
- Thinning of eyebrows
- Menstrual disorders
Signs and tests
A physical exam usually reveals a small thyroid gland. Vital signs (temperature, pulse, rate of breathing, blood pressure) reveal a slow heart rate, low blood pressure, and low temperature.
A chest x-ray may reveal an enlarged heart.
Laboratory tests to determine thyroid function include:
- Free T4 test
- Total T3
- Serum TSH - Results are generally low in secondary hypothyroidism because the pituitary is damaged. However, normal or even high values may be seen.
Additional laboratory abnormalities may include:
- Increased cholesterol levels
- Increased liver enzymes
- Increased serum prolactin
- Low serum sodium
- Low blood glucose
- A CBC that shows anemia
- Deficiency or excess of other pituitary hormones
Imaging will include an MRI of the pituitary to look for a tumor.
The purpose of treatment is to replace the deficient thyroid hormone. Levothyroxine is the most commonly used medication. The lowest dose effective in normalizing thyroid function is used. Life-long therapy may be necessary. Medication must be continued even when symptoms subside.
After replacement therapy has begun, report any symptoms of increased thyroid activity (hyperthyroidism), such as restlessness, rapid weight loss, and sweating.
A high-fiber, low-calorie diet and moderate activity will help relieve constipation and promote weight loss if weight was gained during the time when thyroid activity was low.
In individuals with accompanying hypoadrenalism, steroid replacement must be instituted before thyroid replacement is begun.
In patients who have hypothyroidism caused by a pituitary tumor, surgery may be required. However, surgery may not cure the hypothyroidism, and thyroid replacement will still be needed.
Myxedema coma is treated by intravenous (IV) thyroid replacement and steroid therapy. Supportive therapy of oxygen, assisted ventilation, fluid replacement, and intensive care nursing may be indicated.
With early treatment, return to the normal state is usual. However, relapses will occur if the medication is not continued. Myxedema coma can result in death.
Myxedema coma, the most severe form of hypothyroidism, is rare. It may be precipitated by an infection, illness, exposure to cold, or certain medications.
Symptoms and signs of myxedema coma include:
- Decreased breathing
- Low blood pressure
- Low blood sugar
- Below-normal temperature
Other complications include:
- Heart Disease
- Miscarriage in pregnant women
- Adrenal crisis, if thyroid replacement is begun prior to steroids in hypoadrenal patients
Calling your health care provider
Call your health care provider if signs of hypothyroidism are present, or if chest pain or rapid heartbeat occur.
Call your provider if restlessness, rapid weight loss, sweating, or other symptoms occur after beginning treatment for this disorder.
Call your provider if headache, visual loss, or breast discharge occur.
This condition may not be preventable. Awareness of risk may allow early diagnosis and treatment.
by Harutyun Medina, M.D.
All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.