Thyroid hormones, active or passive transport into the cell?

[HopefulMDclass2020]

A medical student taking an important exam notes that her heart rate is significantly elevated due to thyroid activity. According to the experiment conducted in the passage, which of the following changes would NOT be expected in this student?

Increased cellular hydrolysis of ATP

Increased rate of gluconeogenesis

Increased passive transport of thyroid hormone though cell membranes

Increased translation of PGC-1α protein

C is correct. Thyroid hormone is a tyrosine-based hormone. Contrary to many other lipophilic structures, thyroid hormones cannot traverse cell membranes in a passive manner. The iodine in the o-position makes the phenolic OH-group more acidic, resulting in a negative charge at physiological pH.

Hmm I thought that even through thyroid hormones were derived from a peptide, they passively diffuse into the cell unlike catecholamines. Thoughts?

PS - The answer is not related to the passage, so I figure y'all won't need it. Thanks!

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[basophilic]

Yeah that sounds a bit odd; iodine isn't even that electronegative. Also, I remember learning that thyroid hormone is peptide-derived but has effects almost identical to steroids. But apparently it does have physiological pKa: a quick google search reveals that it's around 6.73 for T4 and 8.45 for T3
The only reason I would have chosen C is that if you already have increased thyroid activity, then the already-present TH would have the consequence of decreasing passive transport over time due to smaller concentration gradient

 

[RH8448]

The point of this question is that thyroid hormone is transported into the cell through active transport which means ATP is used to transport the hormone into the cell. I think this information is out of scope but others may disagree with me as the outline is very general.

You do need to know that thyroid hormone sets basel metabolic rate (relating thyroid hormones to test taking conditions doesn't make much sense to me because your stress hormones would increase rather than your metabolic hormones). Thyroid hormone is hydrophobic - it needs a protein to traverse through the bloodstream. Tyrosine is hydrophobic but the iodine is a part of that molecule. Yes, iodine is still relatively electronegative. Remember iodine can be used as a grignard reagent? So the proton is more likely to fall off making the molecule charged and unable to cross the plasma membrane. As far as entering the nuclear membrane, that must be out of scope as well. But, it does interact with DNA directly to control expression.

So it interacts with DNA like steroids so in that way they are similar.

 

[HopefulMDclass2020]

Well, I did choose C, just out of shear process of elimination. So is the moral of this question that, thyroid hormones act at the DNA level similar to hydrophobic based hormones, however, get transported into the cell via the same process of hydrophilic hormones? I could have sworn I learned it diffused across the membrane in my cell phy class, however, I have been wrong before lol

 

[Quinoline]

So is the moral of this question that, thyroid hormones act at the DNA level similar to hydrophobic based hormones, however, get transported into the cell via the same process of hydrophilic hormones?

This statement is what I was taught.

Something that I find interesting about thyroid hormone is that it is said to be used to increase body temperature by increasing ATP hydrolysis, which causes mitochondria to increase ATP production. However, brown fat is also known to be used for heat generation, but brown fat uses and depletes mitochondria's proton gradient without ATP synthesis for heat generation.

 

[HopefulMDclass2020]

This statement is what I was taught.

Something that I find interesting about thyroid hormone is that it is said to be used to increase body temperature by increasing ATP hydrolysis, which causes mitochondria to increase ATP production. However, brown fat is also known to be used for heat generation, but brown fat uses and depletes mitochondria's proton gradient without ATP synthesis for heat generation.

I think the increased ATP production is from the increased activity of Na/K ATPase by thyroid hormones. I may be wrong though.