Hypothyroidism
Hypothyroidism is a condition that results from inadequate production or action of thyroid hormone, most commonly due to primary hypothyroidism, or failure of the thyroid gland itself. Hypothyroidism can be overt, with a frank decrease in serum thyroxine (T4) levels and a compensatory increase in thyroid-stimulating hormone (TSH) levels. More commonly seen is subclinical hypothyroidism (also called mild thyroid failure), in which the TSH is mildly elevated but T4 levels are normal. Subclinical hypothyroidism often manifests with few or no symptoms, but hypercholesterolemia and subtle cardiac abnormalities can be seen.
2. How common is hypothyroidism?
Hypothyroidism is a common condition. The prevalence of overt hypothyroidism in the United States is estimated at 0.3% to 0.4%, whereas that of subclinical hypothyroidism is much higher (4%-8%). The mean age at diagnosis is the mid-50s. Hypothyroidism is much more common in women, with a female-to-male ratio of 3:1. Postpartum hypothyroidism, a transient hypothyroid phase after pregnancy, occurs in 5% to 10% of women.
3. What are the two most common causes of hypothyroidism?
Although many disorders can cause hypothyroidism, the two most common causes are chronic lymphocytic thyroiditis (Hashimoto’s disease), an autoimmune form of thyroid destruction, and radioiodine-induced hypothyroidism after treatment of Graves’ disease (autoimmune hyperthyroidism).
4. List the less common causes of hypothyroidism.
External irradiation to the neck
Central (pituitary/hypothalamic) hypothyroidism (Fig. 34-1)
Endemic (iodine-deficient) goiter, which is fairly common outside the United States
5. List the symptoms commonly experienced in hypothyroidism.
Hypothyroidism commonly manifests with nonspecific symptoms, such as fatigue, cold intolerance, depression, weight gain, weakness, joint aches, constipation, dry skin, hair loss, and menstrual irregularities.
6. What findings on physical examination are consistent with hypothyroidism?
Physical findings may be normal with mild thyroid failure and should not deter further workup if clinical suspicions are high. Common signs of moderate to severe hypothyroidism include:
Hypertension (diastolic hypertension is a clue)
7. What does palpation of the thyroid reveal?
The thyroid may be enlarged, normal, or small in hypothyroidism, but thyroid consistency is usually firm.
8. Summarize unusual presentations of hypothyroidism.
Unusual presentations of hypothyroidism include megacolon, cardiomegaly, pericardial effusion, and congestive heart failure (CHF). Severe CHF in one reported patient scheduled for cardiac transplant resolved with thyroid hormone replacement alone.
9. Describe the laboratory values that may be abnormal during hypothyroidism.
Laboratory clues to hypothyroidism include normochromic, normocytic anemia (menstruating women may also have iron deficiency anemia due to excessive bleeding from irregular menses), hyponatremia, hypercholesterolemia, and elevations of creatine phosphokinase.
10. What tests best confirm the diagnosis of hypothyroidism in the outpatient setting?
Many thyroid function tests are available to the clinician, including assessments of TSH, T4, triiodothyronine (T3), resin uptake, free T4, free T3, and reverse T3. In the outpatient setting only one test is usually necessary: assessment of TSH. TSH, which is synthesized and secreted from the anterior pituitary gland, is the most sensitive indicator of thyroid function in the nonstressed state. Basically, if the TSH level is normal (range: 0.5-5 mU/L), the patient is euthyroid; if the TSH is elevated (> 5 mU/L), the patient has primary gland failure. In the unusual case in which central hypothyroidism is suspected, a free T4 measurement is the best screening test.
11. How should total T4 levels be interpreted?
