Membrane receptors with cAMP versus intracellular receptors: which hormones use which? Thyroid hormone looks like an exception
Ishita GoyalDropper Sample
28 Sept 2026 · 1 min read
I like to have everything in a table, and the mechanism of hormone action is the one section where I can't fill in the rows confidently.
What I've got: hormones act by binding to specific protein receptors on target cells. There are two kinds. Membrane-bound receptors don't let the hormone enter the cell; the binding generates second messengers like cAMP, IP₃ and Ca²⁺ that regulate the cell's metabolism. Intracellular receptors (mostly nuclear) bind hormones that enter the cell, and the hormone–receptor complex interacts with the genome to regulate gene expression.
NCERT's table of hormone classes: peptide/polypeptide/protein hormones, steroids, iodothyronines, amino acid derivatives. And then it says: hormones which interact with membrane-bound receptors include peptide, polypeptide, protein hormones and amino-acid derivatives, while steroid hormones and iodothyronines interact with intracellular receptors.
So here's what bothers me:
- Thyroid hormones are made from the amino acid tyrosine, so shouldn't they be "amino acid derivatives" and act on membrane receptors? Instead NCERT puts them in a separate group with steroids.
- Is the hormone itself or cAMP called the "second messenger"?
- What is the actual reason that steroid hormones go in the cell and protein hormones don't?
- Which response is faster, and why?
Please correct me wherever my understanding is off.
8 answers & replies
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[+][–]Siddharth RaoDropperSample· 22d ago
Doubt 2 first, as it's the most basic and the most tested: the hormone is the first messenger, because it carries the message to the cell. The second messengers (cAMP, IP₃, Ca²⁺) are made inside the cell after the hormone binds the membrane receptor. A typical cAMP pathway: hormone binds the receptor → G-protein activated → adenylate cyclase activated → ATP converted to cAMP → cAMP activates protein kinase → enzymes get phosphorylated → the cell's response. Because each step multiplies the signal, one hormone molecule can lead to a very large cellular response (signal amplification).
[+][–]Tushar PawarDropperSample· 22d ago
JEE-repeater here, but this is basically a callback chain with a multiplier: one event handler fires, and each step triggers hundreds more. Signal amplification = the cheapest way to make a loud output from a tiny input 😂
[+][–]Tanvi Deshmukh12thSample· 22d ago
Table!! ✨
Chemical nature Examples Receptor location Peptide, polypeptide, protein hypothalamic and pituitary hormones, insulin, glucagon, parathyroid hormone membrane Amino acid derivatives adrenaline, noradrenaline membrane Steroids cortisol, aldosterone, testosterone, estradiol, progesterone intracellular (nuclear) Iodothyronines T₃, T₄ (thyroid hormones) intracellular (nuclear) Your Q3 and Q1 are connected, I'll let the others explain the reason 😉
[+][–]Farhan ShaikhDropperSample· 22d ago
For Q1: both thyroid hormones and adrenaline come from tyrosine, so they're both amino acid derivatives, and since adrenaline works through the cell membrane, thyroid hormones should too. Also I think cAMP is the first messenger and the hormone is the second, because cAMP is the one doing work inside. 🤔
[+][–]Rahul DubeyDropperSample· 22d ago
Farhan, two fixes. Hormone = first messenger, cAMP = second (Siddharth explained it above). And on the tyrosine point: yes, both start from tyrosine, but where the receptor is depends on whether the hormone can cross the cell membrane. This is Ishita's Q3 as well:
- Steroid hormones are derived from cholesterol and are lipid-soluble, so they pass through the lipid bilayer and bind receptors inside the cell.
- Thyroid hormones (iodinated tyrosine derivatives) are small and also behave this way: they enter the cell and bind nuclear receptors.
- Protein/peptide hormones are large and water-soluble, they can't cross the membrane. Catecholamines (adrenaline/noradrenaline) are polar and also stay outside and bind membrane receptors.
So being an "amino acid derivative" doesn't decide it. NCERT keeps iodothyronines as a separate class and groups them with steroids for the mechanism. It's the one exception to memorise.
[+][–]Ishita GoyalDropperSample· 22d agoAuthor
Thank you Rahul. The "can it cross the membrane" rule finally gives me a reason instead of a rote list. Updating the table with a "lipid-soluble?" column.
[+][–]Farhan ShaikhDropperSample· 22d ago
Got it, noted both. Thanks Rahul bhaiya. It's hormone=first, cAMP=second, tyrosine isn't the rule, solubility is.
[+][–]Zoya Khan12thSample· 22d ago
Q4, speed. Membrane receptor hormones usually act quickly: the second messenger changes the activity of enzymes already present in the cell, so the response can appear within seconds to minutes (e.g. adrenaline raising the heart rate). Intracellular receptor hormones usually act more slowly and last longer: the hormone–receptor complex goes to the nucleus, changes gene expression, new mRNA and then new proteins have to be made, so it takes hours.
A small add-on about IP₃ since your note mentions it: some membrane receptors activate an enzyme (phospholipase C) that makes IP₃ from a membrane lipid, and IP₃ makes Ca²⁺ be released from stores inside the cell, which then works as another second messenger.
