Hypothyroidism is a disorder in which the thyroid fails to secrete an adequate secretion of thyroxine (T4) hormone by the thyroid gland. It adversely affects metabolic processes, digestion, and other important functions of the body. The classic lab finding for hypothyroidism is a high TSH with a low free T4 level. Hashimoto’s thyroiditis is the most common cause of hypothyroidism in the United States. Hashimoto’s thyroiditis is a chronic autoimmune disorder in which the body produces destructive antibodies against the thyroid gland. Most patients develop a goiter. Other common causes of hypothyroidism are postpartum thyroiditis and thyroid ablation with radioactive iodine (in treatment for hyperthyroidism).
Thyroid hormone summary
- T4/T3 are released from the thyroid gland in response to circulating serum levels of TSH secreted by the pituitary gland.
- In turn TSH is secretion is influenced by thyroid releasing hormone (TRH).
- TRH is secreted by the hypothalamus.
- There is an inverse relationship between serum level of T3/T4 & TSH/TRH.
- When T3/T4 rise, TSH/TRH secretions are suppressed.
- TRH/TSH levels can be measured directly.
- Elevated TSH and low circulating levels of free (unbound) T3/T4 is diagnostic of Primary hypothyroidism.
- Low or undetectable TSH with high circulating levels of free T3/T4 is diagnostic of thyrotoxicosis.
- T3/T4 have a high affinity for protein. Only unbound is metabolically active.
- T4 is converted into T3 in the peripheral tissue through the removal of iodine. Therefore it is only necessary to give T4 because the body will produce T3 from T4.
3 major thyroid-binding proteins
- Thyroid hormone–binding globulin
Carries approximately 70% of T4 and T3
- Thyroxine-binding prealbumin
Binds approximately 10% of circulating T4 and lesser amounts of T3
Binds approximately 15% of circulating T4 and T3
- Most common form caused by failure within the gland.
- Results from diseases or treatments that destroy thyroid tissue or interfere with thyroid hormone synthesis.
- Primary: elevated TSH with low circulating levels of free (unbound) T3/T4.
- Results from hypothalamic or pituitary disease.
- Low TSH with low T3/T4 OR
- High TSH with high T3/T4
- Do TRH test if suspected secondary hypothyroidism
- Subclinical hypothyroidism is define as an elevated TSH with normal serum free T4
- Failure of the thyroid gland to produce sufficient thyroid hormones. Usually due to fetal exposure to iodine, maternal antibodies, or maternal anti-thyroid drugs.
- Thyroid hormone is essential for normal growth and brain development, almost half of which occurs during the first 6 months of life.
- If untreated, congenital hypothyroidism causes mental retardation and impairs physical growth.
- The manifestations of untreated congenital hypothyroidism are referred to as cretinism.
Clinical Manifestations of Hypothyroidism (more common in women)
- Weight gain
- Dry skin
- Yellowing of skin
- Course hair
- Delayed deep tendon reflexes
- Memory impairment
- Irregular of heavy menses
- Cold intolerance
- Decreased concentration
- Myxedema coma (if not treated)
Order TSH first and, if elevated, order free T3 and T4. Order TSH-receptor antibodies to confirm Hashimoto’s thyroiditis
- TSH elevated (> 6.0 mlU/L) with low serum free T4 = hypothyroidism
- Antimicrosomal antibodies elevated = Hashimoto’s thyroiditis (Gold Standard for diagnosing Hashimoto’s thyroiditis)
Thyroid gland tests
- Thyroid gland ultrasound = used to detect goiter, nodules, and solid versus cystic masses
- Thyroid scan = shows activity of thyroid gland
- Cold spot = not metabolically active. Must rule out thyroid cancer
- Hot spot = metabolically active nodule and is usually benign
- Thyroid cancer = a painless nodule greater then 2.5 cm
- Fine-needle aspiration biopsy = can be done for palpable nodules larger then 1.5 cm. Safe and inexpensive.
- Radionuclide scans = to rule out malignant versus benign nodule
- Physical exam of thyroid gland = to palpate nodules
- CT and MRI scans
- Assessment of thyroid autoantibodies
- Measures of TSH, T3, T4
- Resin uptake test
- Levothyroxine (Synthroid) 25-50 mcg per day is the treatment of choice for the routine management of hypothyroidism
- Levothyroxine is a synthetic preparation of thyroxine (T4). Half-life is 7 days
- Start elderly patients and patients with a history of heart disease with lowest dose
- Increase Synthroid dose by 25 mcg every few weeks until TSH is normalized
- Recheck TSH every 6 to 8 weeks until TSH is normalized
- Given the narrow and precise treatment range for levothyroxine therapy, it is preferable to maintain the patient on the same brand throughout treatment
- Teach patient to report palpitations, nervousness, or tremors. These symptoms may indicate that the thyroid dose is too high
- Elevated TSH level with a normal free T4 level is most consistent with subclinical hypothyroidism
- The most common cause of hypothyroidism is autoimmune thyroiditis
- The T4 dose needed by elderly patients is 75% or less of that needed by younger adults
- One physical examination finding in hypothyroidism is a delayed relaxation phase of the deep tendon reflex
- Hypertriglyceridemia is likely to be found in a person with untreated hypothyroidism
- A fixed, painless thyroid mass accompanied by hoarseness and dysphagia should raise the suspicion a thyroid malignancy
- Possible consequences of excessive levothyroxine use include bone thinning
- The correct procedure for palpation of a thyroid gland is to have the client relax the sternocleidomastoid muscles, which is done by having the client lower the chin and lean head toward the side being evaluated
- During the examination of the thyroid gland, when a client takes a sip of water and swallows, the thyroid gland moves upward during the swallow and feels symmetric and smooth to palpation
- Calcium supplements, antacids, ferrous sulfate, and aluminum hydroxide decrease the effectiveness of levothyroxine (Synthroid).
- It takes 2-3 weeks to see hypothyroidism improvement in after taking levothyroxine.
- Insulin and digoxin levels need to be increased when converting a patient from a hypothyroid to a euthyroid state.