Lippincott Nursing Pocket Card - November 2023

Understanding Hypothyroidism and Hyperthyroidism

The thyroid gland produces and secretes hormones which regulate metabolism and body temperature as well as promote normal growth and development in children. Disorders of thyroid function are common in primary care and a contributing factor in critical illness and psychiatric disease. Both hypothyroidism and hyperthyroidism can be transient or permanent depending on the underlying cause.

Pathophysiology

The thyroid gland is part of the hypothalamus-pituitary-thyroid axis and is controlled by a negative feedback loop. The hypothalamus secretes thyroid releasing hormone (TRH) which stimulates the anterior pituitary gland to secrete thyroid stimulation hormone (TSH) which in turn stimulates the thyroid to release thyroid hormones, thyroxine (T₄) and triiodothyronine (T₃). These hormones control how the body uses energy, so they affect nearly every organ in your body. When blood levels of thyroid hormones increase, TRH and TSH are inhibited.

Hypothyroidism

Hypothyroidism is a state of low circulating thyroid hormones. Primary causes for this condition, which affect the thyroid gland directly, impair its ability to make enough hormone. Rarely, there may be a secondary cause such as a pituitary gland tumor, which blocks the pituitary production of TSH. Hypothyroidism results in the slowing down of many physical and mental processes. Myxedema coma, the decompensated severe form of hypothyroidism, is a medical emergency which warrants immediate treatment with thyroid hormone and intensive care unit admission (Ross, 2023a).

Causes of Hypothyroidism

Primary Hypothyroidism
- Hashimoto disease (autoimmune thyroiditis)
- Iodine deficiency
- Surgical removal of the thyroid gland
- Thyroid ablation with radioactive iodine
- External radiation
- Temporary inflammation of thyroid gland
- Too little thyroid medication
- Exposure to some pharmacologic agents (i.e., amiodarone, lithium, thalidomide)
Secondary Hypothyroidism
- Pituitary-based deficiencies of TRH-stimulating or TRH-releasing hormone
- Rare disorders related to impaired stimulation of the thyroid gland by the pituitary
- Pituitary adenoma

Signs and Symptoms of Hypothyroidism (Surks, 2022)

General loss of energy
Slowed metabolism resulting in fatigue, slow movement and speech, cold intolerance, hypothermia, slowed deep tendon reflexes
Skin: cool, pale, sweating, course hair, hair loss, brittle nails, nonpitting edema (myxedema)
Eyes: periorbital edema and Graves’ eye disease (eyes appear to bulge out)
Hematology: anemia
Cardiovascular: bradycardia, decreased cardiac output
Respiratory: shortness of breath on exertion, rhinitis
Gastrointestinal: constipation, decreased taste sensation, weight gain
Hypercholesterolemia
Reproductive: menstrual irregularities/infertility in women, decreased libido
Neurologic: mild cognitive impairment, Hashimoto encephalopathy, myxedema coma
Musculoskeletal: weakness, cramps, myalgias
Stunted growth in children

Treatment of Hypothyroidism
Thyroid replacement, using a form of T₄ (levothyroxine sodium), is the preferred treatment that requires only once a day dosing. Thyroid replacement should be tailored for each patient as small changes in dose can quickly cause an imbalance. Patients should be monitored closely until stable. After TSH is stabilized, maintenance therapy should be continued with annual or semiannual TSH testing.

Clinical Considerations

Inform patient that thyroid replacement treatment will likely continue for life.
Recheck the patient’s TSH level if the brand of medication changes.
Advise patient that medication should be taken at the same time each day on an empty stomach (at least one hour before other medications).
Teach patient that reduced absorption can occur when taken with calcium, iron, vitamins, antacids, colestipol (or other medications that bind bile acids) and fiber supplements.
Screen patients with depression, anxiety, and cognitive problems for thyroid disease.

Hyperthyroidism

Hyperthyroidism is a state of high circulating thyroid hormones due to overproduction by the thyroid gland or other sources. Thyrotoxicosis is the clinical syndrome that results from excess thyroid hormone with thyroid storm being the most severe form. Thyroid storm, a rare diagnosis, can occur in patients with or without preexisting hyperthyroidism and is usually triggered by factors such as trauma, myocardial infarction, surgery (including thyroid surgery for hyperthyroidism), or infection. Thyroid storm is a medical emergency, warranting immediate administration of medications to block the production and release of thyroid hormone, and management of systemic illness (Ross, 2023b).

Causes of Hyperthyroidism

Graves’ Disease
Thyroid tumor/nodule
Excessive intake of thyroid hormones
Abnormal secretion of TSH
Thyroiditis
Excessive iodine intake

Signs and Symptoms of Hyperthyroidism (Ross, 2022)

Skin: warm, sweaty, hyperpigmentation, pruritus, hives, loose and soft nails, thinning or loss of hair
Eyes: lid lag, exophthalmos, impairment of eye muscles, periorbital and conjunctival edema
Hematology: immune thrombocytopenia, pernicious anemia, increased prothrombotic factors (factors VIII, IX, fibrinogen, plasminogen activator inhibitor-1)
Cardiovascular: tachycardia, hypertension, widened pulse pressure, atrial fibrillation
Respiratory: dyspnea and dyspnea on exertion
Gastrointestinal: weight loss, increased metabolic rate, vomiting, diarrhea, dysphagia
Metabolic: bone density loss, osteoporosis, hyperglycemia, low serum cortisol
Genitourinary: urinary frequency, nocturia
Reproductive: amenorrhea
Neuropsychiatric: anxiety, nervousness, restlessness, irritability, emotional lability, insomnia, psychosis, agitation, depression, confusion, poor concentration
Musculoskeletal: tremors in the hands, weakness
Heat intolerance
Goiter (enlarged thyroid)

Treatment of Hyperthyroidism
Hyperthyroid treatment includes the following:

Medications:
- Beta blockers: alleviate symptoms (palpitations, tachycardia, tremors, anxiety)
- Thionamides (antithyroid drugs): methimazole and propylthiouracil reduce thyroid hormone synthesis; may suppress white blood cell production
Radioactive iodine: one-time oral treatment ablates the hyperactive gland
- A major complication of this treatment is permanent hypothyroidism.
Surgery to remove the thyroid tissue that is producing the excessive thyroid hormone.
- A major complication is disruption of the surrounding tissue, including accidental removal of the parathyroid glands, resulting in low calcium levels requiring calcium replacement.

Special Populations

Pregnancy
Thyroid dysfunction in pregnancy is associated with preeclampsia, spontaneous abortion, abnormal fetal brain development, and fetal mortality. In general, increased dosage requirements of thyroid replacement medications should be anticipated, especially during first and second trimesters.

Children
Thyroid hormones play a critical role in neurologic development in children. Low or absent levels of thyroid hormone may result in cretinism, and neonatal hypothyroidism accounts for the most preventable cause of intellectual disability.

Elderly
Signs and symptoms of hypothyroidism may be very subtle and mistakenly attributed to normal aging changes. Taking a careful history is important to make the correct diagnosis and helps to avoid erroneous diagnoses of heart failure, dementia or depression.

Thyroid Function Tests (TFTs) (Ross, 2023c)

Thyroid function tests (TFTs) are used to screen thyroid activity. TSH is the gold standard for evaluating thyroid function.

When screening for thyroid dysfunction:
     • If the TSH is normal, no further testing is required.
     • If the TSH is high, check free T4 to determine the degree of hypothyroidism.
     • If the TSH is low, check free T4 and T3 to determine the degree of hyperthyroidism.
     • If pituitary or hypothalamic disease is suspected, check both serum TSH and free T4.
     • If TSH is normal, but patient has convincing symptoms of thyroid dysfunction, check free T4.

Normal Thyroid Test Values
Laboratory Test	Normal Range (Adult)
TSH	0.4 to 5.0 mIU/L
Total T₄	4.6 to 11.2 mcg/dL
Free T₄	Varies with methodology used.
Total T₃	75 to 195 ng/dL
Free T₃	Varies among laboratories.

Note: TSH levels peak in the evening and are at their lowest in the afternoon. Also, levels may vary with severe stress, illness, trauma, and low energy intake.

References:
Ross, D. (2023a, April 10). Myxedema coma. UpToDate. https://www.uptodate.com/contents/myxedema-coma

Ross, D. (2023b, September 29). Thyroid storm. UpToDate. https://www.uptodate.com/contents/thyroid-storm

Ross, D. (2023c, September 20). Laboratory assessment of thyroid function. UpToDate. https://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function

Ross, D. (2022, February 15). Overview of the clinical manifestations of hyperthyroidism in adults. UpToDate. https://www.uptodate.com/contents/overview-of-the-clinical-manifestations-of-hyperthyroidism-in-adults

Surks, M.I. (2022, August 29). Clinical manifestations of hypothyroidism. UpToDate. https://www.uptodate.com/contents/clinical-manifestations-of-hypothyroidism

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Understanding Hypothyroidism and Hyperthyroidism