USMLE® Step 1 style questions USMLE
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A 34-year-old woman comes to clinic because of tearing and foreign body sensation of her eyes bilaterally, which has gradually worsened over the last several weeks. She also notes having occasional palpitations, nervousness, sweating, and heat intolerance. Her past medical history is unremarkable. She is currently a daily smoker. Physical examination shows an anxious, trembling woman. She has eyelid retraction bilaterally, with an inability to fully close her eyes. Her extraocular motility is limited on upgaze. There is no thyromegaly or thyroid nodules noted. Laboratory studies reveal a thyroid-stimulating hormone level of 0.1 μU/mL and thyroxine (T4) 42 μg/dL. Thyroid stimulating immunoglobulin is positive. CT scan of the orbits shows proptosis and marked enlargement of the extraocular muscle, with sparing of the tendons. Which of the following would be likely to transiently worsen this patient’s eye symptoms?
Contributors:Yifan Xiao, MD
Normally, the hypothalamus, which is located at the base of the brain, detects low blood levels of thyroid hormones and releases thyrotropin-releasing hormone into the hypophyseal portal system - which is a network of capillaries linking the hypothalamus to the anterior pituitary.
The anterior pituitary then releases thyroid-stimulating hormone, also called thyrotropin or simply TSH.
TSH stimulates the thyroid gland which is a gland located in the neck that looks like two thumbs hooked together in the shape of a “V”.
The thyroid gland is made up of thousands of follicles, which are small spheres lined with follicular cells.
Follicular cells convert thyroglobulin, a protein found in follicles, into two iodine-containing hormones, triiodothyronine or T3, and thyroxine or T4.
Once released from the thyroid gland, these hormones enter the blood and bind to circulating plasma proteins.
Only a small amount of T3 and T4 will travel unbound in the blood, and these two hormones get picked up by nearly every cell in the body.
Once inside the cell T4 is mostly converted into T3, where it can exert its effect. T3 speeds up the cell’s basal metabolic rate.
It’s as if the cells are in a bit of frenzy.
T3 increases cardiac output, stimulates bone resorption - thinning out the bones, and activates the sympathetic nervous system, the part of the nervous system responsible for our ‘fight-or-flight’ response.
Thyroid hormone is important - and the occasional increase can be really useful when you need a boost to get through the final rounds of a sporting competition or when you’re trying to stay warm during a snowstorm!
Now, hyperthyroidism can happen a few different ways - all of them result in too much thyroid hormone and a hypermetabolic state, where cellular reactions are happening faster than normal.
These autoantibodies include thyroid-stimulating immunoglobulins, which bind to the TSH receptor on follicular cells and imitate TSH.
This results in growth of the thyroid gland and stimulates the follicular cells to produce excess thyroid hormone.
Another primary cause is toxic nodular goiter, where one or more follicules start generating lots of thyroid hormone - in some cases it’s because of a mutated TSH receptor that inappropriately keeps these follicular cells active.
A different cause is a hyperfunctioning thyroid adenoma, where the follicular cells start growing uncontrollably forming a benign tumor and making excess thyroid hormones.
Also, anytime the thyroid gets inflamed or damaged, there can be a large release of pre-formed thyroid hormones.