Active learning 7 - HPT Axis Key PDF

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BIOL 171A Winter 2021 Active Learning Exercise 7 – Feb. 26, 2021 Group members:

Barbara, a 46-year-old woman, is sitting in the waiting room of her physician’s office. Barbara is returning for an annual checkup after her recent battle with breast cancer. It seems as if the cancer treatment has been successful and Barbara is now cancer-free. During the examination, the doctor finds nothing to suggest that any tumors have returned. However, Barbara has not been feeling well lately and is concerned. She tells the doctor that she has been feeling abnormally cold lately and that she has also gained about 20 pounds over the past few months. Along with this, lately Barbara has suffered from overall fatigue. Barbara insists that she has not changed her activities or diet and has no idea why the sudden changes have occurred. The physician feels Barbara’s neck and doesn’t find any signs of swelling. He then orders a series of blood tests. The results of these tests are listed below. Use this information, along with the test results, your book, and anything you have learned in lecture, to answer the following questions.

Test Serum thyroxine (T4) (ug/dl) Thyrotropin (TSH) (μIU/mL) Thyroid antibodies Serum Triiodothyronine (T3) (ng/dl) Blood Pressure Hematocrit (%) Hemoglobin (g/100 ml blood) Glucose (mg/dl) Sodium (mmol/L) Potassium (mmol/L)

Result 2.2 0.2 Normal 33 95/55 38 13.4 92 137 4.2

Normal Range 4.6-12 0.5-6 varies 80-180 90-120/60-80 36-38 11-14 70-110 135-145 3.5-5.0

LOW LOW LOW

1. List some common symptoms of hypothyroidism and hyperthyroidism in adults. Is Barbara’s thyroid functioning properly? If not, is she suffering from hypo- or hyperthyroidism? Common effects of hypothyroidism include decreased metabolic rate, cold intolerance, brittle nails, thin hair and skin, fatigue, slow reflexes, slowing of speech and thought processes. Common symptoms of hyperthyroidism include increased metabolic rate, heat intolerance, muscle weakness, weight loss, increased heartbeat, and increased force of contraction in the heart. (Also bulging eyes, if the cause is Graves’ disease) Barbara’s thyroid is not functioning properly. Based on the endocrine data and description of symptoms above, Barbara has hypothyroidism. Hypothyroidism simply means producing too little of the thyroid hormones (T3 and T4).

2. What are some possible causes of low levels of T3 and T4? Draw a diagram of a normal hypothalamicpituitary-thyroid (HPT) axis and refer to the diagram to explain your answer.

Potential causes of low T3 and T4 include 1) Low TRH from the hypothalamus and/or low TSH from the anterior pituitary. The decrease in TSH and/or TRH could be from a tumor. 2) Iodine deficiency (primary hypothyroidism – would get goiter) or Hashimoto’s Disease (autoimmune disease where antibodies attack various parts of the thyroid itself – goiter or no goiter, depends). 3) Exposure to radioactive iodine could also destroy thyroid cells, thus causing a decrease in T3 and T4.

3. What are some possible causes of low levels of TSH? Refer to the HPT diagram to explain your answer. Low levels of TSH can be caused by intense negative feedback from high levels of T3 and T4 (not the case here, as T3 and T4 levels are low) or a tumor that causes a decreased output.

4. What do you think is the underlying cause of all these symptoms in Barbara? (i.e., What do you think has gone wrong with her HPT axis?) Explain your answer. The most likely cause is a tumor that suppresses either TRH or TSH secretion. It’s almost impossible to tell which one since we can’t really tap into the portal blood supply and measure TRH concentrations (most patients won’t let you cannulate their brain or pituitary). Barbara should go for an MRI to see if a tumor is visible. Since she had cancer previously, it may have migrated (although tumors associated with dysregulated thyroid hormones can be malignant or benign).

5. Barbara does not have a goiter at the moment. If her condition continues unchecked will she develop one? Why or why not? Refer to the HPT diagram to explain your answers. Elevated secretion of TSH or thyroid-stimulating antibodies over a long period of time causes excessive growth of the thyroid and therefore causes development of a goiter. Barbara is hypothyroid with low levels of TSH and normal antibody levels. Therefore, her thyroid is not being overly stimulated, and she will not develop a goiter....