Hypothyroidism Basics for the Physician Assistant Exam
The thyroid is a vital organ that the Physician Assistant Exam (PANCE) will expect you to be familiar with. One of its big responsibilities is regulating the basal metabolic rate. If your thyroid gland becomes lethargic, you may have problems with weight gain and extreme fatigue.
Hypothyroidism refers to a thyroid that’s underactive. Here are four possible causes:
Hashimoto’s thyroiditis is an autoimmune condition. It’s the most common cause of hypothyroidism, and it mainly affects women around their 30s.
DiGeorge syndrome (DGS) is a hereditary cause of hypothyroidism. The syndrome causes not only thyroid and parathyroid problems but also heart problems, developmental delays, and hypocalcemia. A person with DiGeorge syndrome may also have a cleft palate problem.
If you see a question on the PANCE about non-autoimmune causes of hypothyroidism, look for an answer that includes prior neck surgery, usually from prior thyroid or parathyroid surgery.
Commonly prescribed medications that can cause hypothyroidism include amiodarone (Cordarone) and lithium (Lithobid).
Note that if someone’s on steroids, secretion of thyroid-stimulating hormone (TSH) can decrease. For someone in the critical care unit, the medication dopamine can also lower TSH secretion. The key is to recheck thyroid function tests when the patient is off these medications.
The symptoms of hypothyroidism often depend on how underactive the thyroid gland is. They can include hoarseness, profound weakness and fatigue, weight gain, cold intolerance, constipation, and amenorrhea, as well as depression. In fact, part of the initial workup of depression includes screening for hypothyroidism.
Hypothyroidism can cause pain in the muscles (a proximal type of myopathy). Physical examination can reveal loss of the eyebrows (the lateral aspect), and a neck exam can reveal a goiter. There can be bradycardia and elevated blood pressure. Examination of the deep tendon reflexes (especially the Achilles reflex) can show a delayed relaxation response.
There’s a close relationship among the hypothalamus, pituitary gland, and thyroid glands. The hypothalamus secretes thyroid releasing hormone (TRH), which stimulates the pituitary to make thyroid stimulating hormone (TSH), which finally stimulates the thyroid to make thyroxine (T4). Continue to follow the logic here:
Primary hypothyroidism (a problem of the thyroid) is characterized by a low free T4 and an elevated TSH.
Secondary hypothyroidism (a problem with the pituitary) is characterized by a low TSH and a low free T4.
Tertiary hypothyroidism, which is not common clinically, is caused when the hypothalamus doesn’t make TRH.
The treatment for hypothyroidism is oral thyroid replacement. The usual treatment is levothyroxine (Synthroid). After beginning this medication, it’s customary to wait about 6 weeks before doing a follow-up TSH level.
An emergent medical condition resulting from long-standing untreated hypothyroidism is myxedema coma. This condition is characterized by extreme lethargy, confusion, and hypothermia. There’s often significant total body edema. On blood work, hyponatremia may be present. Lab values often show a very elevated TSH level. The treatment is usually thyroid replacement, most often given intravenously. Intravenous steroids are often given first before the thyroid replacement.
Which of the following is characteristic of Hashimoto’s thyroiditis?
(A) Serum high T4 and low TSH
(B) The presence of antimicrosomal antibodies
(C) Association with atrial fibrillaton
(D) No association with other autoimmune conditions
(E) Treatment includes radioactive iodine
The answer is Choice (B). Choice (A) is incorrect because it lists the characteristic findings in someone with primary hyperthyroidism, and hyperthyroidism is associated with atrial fibrillation, Choice (C). Both hypothyroidism and hyperthyroidism are associated with other autoimmune conditions. And as for Choice (E), radioactive iodine is used in the treatment of hyperthyroidism, namely Graves’ disease.