Name:
A 23-Year-Old with Undetectable TSH
Description:
A 23-Year-Old with Undetectable TSH
Thumbnail URL:
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Duration:
T00H04M05S
Embed URL:
https://stream.cadmore.media/player/2170b835-a6a6-4b12-a704-07e284be61ef
Content URL:
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Upload Date:
2022-02-28T00:00:00.0000000
Transcript:
Language: EN.
Segment:0 .
[music]
CATHY: Hi, welcome to Harrison's Podclass where we discuss important concepts in internal medicine. I'm Cathy Handy. And I'm Charlie Wiener, and we're coming to you from the Johns Hopkins School of Medicine. Welcome to Harrison's Podclass. This is Episode 3: A 23-Year-Old with an undetectable TSH. Our question: A 23-year-old college student is followed in the student health center for medical management of panhypopituitarism after resection of a craniopharyngioma as a child.
CATHY: She reports moderate compliance with her medications but feels generally well. Thyroid stimulating hormone, TSH is checked and it's below the limits of detection of the assay. Which of the following is the next most appropriate action? a) Decrease levothyroxine dose to half of the current dose. b) Do nothing. c) Order a free T4 level. d) Order an MRI of her brain.
CATHY: or e) order a thyroid uptake scan. So, Cathy, what are your thoughts on this case?
CATHY: The major things to note here are that the patient's young and she's relied on exogenous pituitary hormone since her childhood surgery. She's also been able to live a relatively normal life with diligent management of her hormonal replacement. In addition to thyroid replacement in patients who've had their pituitary removed, it's important to remember that in times of significant physiologic stress like an acute illness, she may also require stress dose steroids.
CHARLIE: What do you make of the undetectable TSH in her case?
CATHY: So, going over the thyroid hormone access loop, you remember that the hypothalamus makes TRH which stimulates the pituitary to make TSH, and then that acts on the thyroid to make T4 and T3. All of these are in a feedback loop so TSH from the pituitary feeds back on the hypothalamus and downregulates the production of TRH, and then, T4 and T3 feed back on the pituitary and downregulate TSH. So, getting back to the question that you asked about the undetectable TSH in this case, since the patient doesn't have a pituitary gland she won't produce TSH, so really, here it's not helpful at all.
CHARLIE: What about utilizing TSH to monitor thyroid function in a normal patient?
CATHY: So, in a normal patient, TSH is what you would use. And we use it mostly to screen for hyperthyroidism or hypothyroidism, or in a patient who's on thyroid replacement because of a primary thyroid disorder. You would also use it to measure the adequacy of thyroid replacement. If TSH is undetectable that means that there's too much thyroid hormone replacement; and if it's elevated, that means that there's not enough hormone replacement. Therefore, the answer in this case is C; that you want to measure the free T4.
CATHY: And you would also want to know if she's having any symptoms related to hyper or hypothyroidism.
CHARLIE: What about the other answers? Is there any role of the imaging in patients such as this?
CATHY: Not in this case. MRI would be useful if you thought that there was a new or a recurrent intracranial process. It may have been used when she initially presented. And a thyroid uptake scan, you would use mostly for a suspicion of thyroid disease such as autoimmune thyroiditis. Neither of those apply in this case.
CHARLIE: That's great, Cathy. So, in this case the answer is C, order a free T4 level because the undetectable TSH is not helpful because of the absence of her pituitary. This is a great teaching point that TSH can be a useful screening test in most patients, but in patients without a functioning pituitary for whatever reason, surgery or otherwise, you must go more downstream and measure the direct measurement of free T4 to measure the adequacy of a thyroid replacement therapy.
CATHY: It's also important to remember that in any patient taking exogenous thyroid hormone replacement, it's important to ask about clinical symptoms in addition to the lab test. And to read more about this, you can check out Harrison's chapter on disorders of the thyroid gland.
CHARLIE: Thank you! [music]