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Understanding Hypothyroidism and Hyperthyroidism

Disorders of thyroid function are one of the most commonly encountered endocrine abnormalities in primary care. 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. Several causes for this condition, most of which affect the thyroid gland directly, impair its ability to make enough hormone. Rarely, there may be a pituitary gland tumor, which blocks the pituitary production of TSH. This results in the thyroid producing too few hormones, slowing down many physical and mental processes.

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 and Tertiary Hypothyroidism

Pituitary-based deficiencies of TRH or TSH
Rare disorders related to impaired stimulation of the thyroid gland at the pituitary level
Pituitary adenoma

Signs and Symptoms of Hypothyroidism:
     • General loss of energy
     • Slowed metabolism
     • Weight gain
     • Bradycardia
     • Dry skin and hair
     • Constipation
     • Cold intolerance
     • Puffy skin
     • Hair loss
     • Altered cognition
     • Hyporeflexia
     • Menstrual irregularities/infertility in women
     • Stunted growth in children

Treatment of Hypothyroidism
Thyroid replacement, using a form of T4 (levothyroxine sodium), is the preferred treatment. It is a more stable form of thyroid hormone and requires only once a day dosing. Thyroid replacement should be tailored for the individual patient because small changes in dose can rapidly shift a patient from a euthyroid state. Patients should be monitored closely until stable. After TSH is stabilized, maintenance therapy should be continued with annual or semiannual TSH testing.

Clinical Considerations

Tell patient that thyroid replacement treatment will likely continue for life.
If the brand of medication changes, recheck the patient’s TSH level.
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 significant interference with absorption can occur when taken with calcium, iron, vitamins, antacids, colestipol or other medications that bind bile acids or fiber supplements.

Hyperthyroidism

Hyperthyroidism is a state of high circulating thyroid hormones. Thyrotoxicosis refers to the state of thyroid hormone excess arising from either overproduction from the thyroid gland or extrathyroidal, including exogenous, sources.

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:
     • Hot flashes, sweating
      • Tachycardia
      • Anxiety, nervousness
      • Weight loss
      • Hair loss
      • Difficulty sleeping, restlessness
      • Tremors in the hands
      • Weakness
      • Diarrhea
      • Emotional instability, irritability or fatigue
      • Goiter • Moist, sweaty skin
      • Exophthalmos, lid lag

Treatment of Hyperthyroidism
There are two main antithyroid drugs available: methimazole and propylthiouracil. It is important to note that these medications can cause occasional suppression of white blood cell production. Medications are also available to immediately treat the symptoms caused by excessive thyroid hormones, for example, a beta-blocker to manage tachycardia.

Radioactive iodine is another option to treat hyperthyroidism. A one-time oral treatment ablates the hyperactive gland. However, permanent hypothyroidism is the major complication of this form of treatment.

Surgical removal of the thyroid tissue that is producing the excessive thyroid hormone is another option. A major complication is disruption of the surrounding tissue, including accidental removal of the parathyroid glands, resulting in low calcium levels and thus 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)

Thyroid function tests (TFTs) are used to screen thyroid activity. Thyroid stimulating hormone (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:

Leung, A. M. (September/October 2016). Thyroid emergencies. Journal of Infusion Nursing, 281-286.

Mccaron, K. C. (2009). Deciphering diagnostics: deciphering thyroid function testing. Nursing Made Incredibly Easy, 11-14.

Ross, D. (2017, November 7). Laboratory assessment of thyroid function. UpToDate. Retrieved from https://www.uptodate.com/contents/laboratory-assessment-of-thyroid-function

Ross. D. (2018). Disorders that cause hypothyroidism. UpToDate. Retrieved from https://www.uptodate.com/contents/disorders-that-cause-hypothyroidism?search=disorders%20that%20cause%20hypothyroidism

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Understanding Hypothyroidism and Hyperthyroidism