Think of your body as a finely tuned engine. Every cell burns fuel, generates heat, and performs work. That engine needs a throttle — something that tells every cell, "speed up" or "slow down." The thyroid gland, a butterfly-shaped organ wrapped around your windpipe, makes that throttle. It produces two hormones, T4 (thyroxine) and T3 (triiodothyronine), that set the metabolic pace of almost every tissue in your body.
But the thyroid does something else too. It also makes a completely different hormone, thyrocalcitonin (calcitonin), that has nothing to do with metabolism. This one is a calcium manager — it helps keep your blood calcium from climbing too high.
So the thyroid is really two glands in one: a metabolic throttle and a calcium brake.
The Metabolic Hormones: T4 and T3
The thyroid follicle cells (the main cells of the gland) take up iodine from your blood and attach it to the amino acid tyrosine. Depending on how many iodine atoms get attached, you get either:
T4 (thyroxine) — four iodine atoms. This is the main product, about 90% of what the thyroid releases.
T3 (triiodothyronine) — three iodine atoms. This is the much more active form, about 10% of release.
Here's the key: T4 is mostly a prohormone. It circulates in the blood, but most cells convert it into T3 right where they need it. T3 then binds to receptors inside the cell nucleus and directly turns genes on or off — ramping up the production of enzymes that control oxygen consumption, heat production, and energy use.
Important
T3 is the active hormone; T4 is the circulating reservoir. Most of the T4 gets converted to T3 in the liver, kidneys, and other tissues.
What does this actually do? A higher T3 level means your cells burn more calories, produce more heat, and work faster. Your heart beats faster, your gut digests quicker, your brain stays alert. Too little (hypothyroidism) and everything slows down — fatigue, weight gain, cold intolerance. Too much (hyperthyroidism) and the engine races — weight loss, sweating, anxiety, rapid pulse.
Now switch gears entirely. Scattered between the thyroid follicles are different cells called parafollicular cells (or C-cells). These make thyrocalcitonin, often just called calcitonin.
Calcitonin's job is to lower blood calcium when it gets too high. It does this by:
Stopping bone breakdown — it tells osteoclasts (the cells that dissolve bone) to slow down, so calcium stays locked in bone.
Increasing calcium loss in urine — the kidneys excrete more calcium.
Note
| Hormone | Source | Target | Effect |
|---------|--------|--------|--------|
| T4 / T3 | Follicle cells | Almost all cells | Increases metabolic rate |
Calcitonin, known as thyrocalcitonin, is produced by the thyroid gland and plays a role in lowering and regulating the level of calcium in the blood.
The thyroid gland is best known for its two hormones, tetraiodothyronine (T4) and triiodothyronine (T3), made by its follicular cells and dependent on dietary iodine. Alongside these, the thyroid also secretes a separate protein hormone, thyrocalcitonin (TCT), whose job is distinct from that of T3 and T4. …
Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.
TG EAPCET 2023Set ap-2023-05-10-AN1 markMCQ
Q.Thyroxine is
(A) Amine hormone
(B) Peptide hormone
(C) Protein hormone
(D) Steroid hormone
›Reveal solutionSolution
Thyroxine is synthesized from the amino acid tyrosine and contains iodine atoms, making it an amine-derived hormone. The answer is (A).
Hormones are classified by their chemical structure, which determines how they're synthesized, transported, and how they interact with target cells. Understanding this classification helps predict a hormone's properties.
The four major classes are:
Amine hormones: derived from single amino acids (tyrosine or tryptophan)
Peptide hormones: short chains of amino acids
Protein hormones: longer polypeptide chains
Steroid hormones: derived from cholesterol, with a characteristic four-ring structure
Thyroxine (T₄) and triiodothyronine (T₃) are thyroid hormones with a unique structure. They're built from two tyrosine molecules that are covalently linked, with iodine atoms attached to the aromatic rings. Despite being relatively small and derived from amino acids, their lipophilic nature (due to the iodinated aromatic rings) allows them to cross cell membranes and bind to intracellular receptors—behavior more typical of steroid hormones.
The key distinguishing feature is the biosynthetic origin. Thyroxine is synthesized by:
Iodination of tyrosine residues on thyroglobulin protein within the thyroid follicles
Coupling of two iodinated tyrosines (DIT + DIT → T₄)
Proteolytic cleavage to release the finished hormone …
Q.Non iodised hormone secreted by the largest endocrine gland is
(A) Thyroxine
(B) Parathormone
(C) Adrenaline
(D) Calcitonin
›Reveal solutionSolution
The largest endocrine gland is the thyroid gland. Among its secretions, Calcitonin is a non-iodised hormone. The correct option is (D).
The question asks us to identify a non-iodised hormone secreted by the largest endocrine gland. To solve this, we first need to know which gland is the largest endocrine gland in the human body and then examine the hormones it produces, specifically looking for one that does not contain iodine.
Identify the largest endocrine gland: The thyroid gland is the largest endocrine gland in the human body. It is located in the neck, anterior to the trachea.
Identify hormones secreted by the thyroid gland: The thyroid gland primarily secretes three hormones:
Thyroxine (also known as T4 or tetraiodothyronine)
Triiodothyronine (also known as T3)
Calcitonin
Determine which thyroid hormones are iodised and non-iodised:
Thyroxine (T4) and Triiodothyronine (T3) are thyroid hormones that are derivatives of the amino acid tyrosine and contain iodine atoms. Their synthesis critically depends on the availability of iodine.
Calcitonin, on the other hand, is a peptide hormone. It is secreted by the parafollicular cells (C-cells) of the thyroid gland. Calcitonin does not contain iodine in its structure. Its primary function is to lower blood calcium levels.
Evaluate the given options based on this information:
(A) Thyroxine: This hormone is secreted by the thyroid gland, but it is an iodised hormone. So, this option is incorrect.
(B) Parathormone: This hormone is secreted by the parathyroid glands (four small glands located on the posterior surface of the thyroid gland), not the thyroid itself. It is a peptide hormone and non-iodised, but it doesn't come from the largest endocrine gland. So, this option is incorrect. …