🦋 Endocrine System Lesson

T3, T4, and Calcitonin: the thyroid hormones

The thyroid produces two hormones controlling metabolism and one controlling calcium — three distinct jobs from one gland.

Storage

Active

Calc.

Ca2+ down

📖 Full Breakdown

Three hormones, and why iodine is central to two of them

The metabolic hormones and the calcium hormone come from entirely different cell types within the same gland.

T4 (Thyroxine)

The main secreted form

Released in the largest quantity, but converted to the more active T3 in peripheral tissues. Both T3 and T4 require iodine to synthesize — iodine deficiency causes goiter (an enlarged thyroid trying to compensate).

T3 (Triiodothyronine)

The active form

3 to 4 times more potent than T4. Most T3 in the body comes from peripheral conversion of T4, not direct thyroid secretion.

Calcitonin

Produced by parafollicular C cells

Lowers blood calcium by inhibiting osteoclasts (bone-resorbing cells) — functionally the opposite of PTH from the parathyroid gland.

🩺 Clinical / Exam Application

A patient in an iodine-deficient region develops a visibly enlarged thyroid (goiter) without any change in thyroid hormone levels. Because both T3 and T4 synthesis absolutely require iodine, insufficient iodine intake means the thyroid can't produce enough hormone no matter how hard it tries — the gland enlarges as it attempts to compensate by growing more tissue, even though this doesn't fix the underlying iodine shortage.

⚠️ Exam Alert

A frequently tested lab pattern: low TSH + high T4 indicates hyperthyroidism (the thyroid is overactive, so the pituitary reduces TSH via negative feedback), while high TSH + low T4 indicates hypothyroidism. Getting this pairing backwards is one of the most common exam mistakes.

🚧 Common Trap

Don't assume calcitonin and PTH come from the same gland just because calcitonin is discussed alongside thyroid hormones. Calcitonin is made in the thyroid (by C cells) but PTH is made in the separate parathyroid glands — they have opposite effects on calcium but come from different organs.

✅ Quick Check

A patient has hyperthyroidism. What would you expect their TSH and T4 lab values to look like, and why?

📝 Exam Prep

Common Exam Questions

❓ What happens in hypothyroidism vs hyperthyroidism?

✅ Hypothyroidism: low T3/T4 causes fatigue, weight gain, cold intolerance, bradycardia, and constipation. Hyperthyroidism: high T3/T4 causes heat intolerance, weight loss, tachycardia, and exophthalmos (in Graves disease).

❓ What is the difference between T3 and T4?

✅ T4 (thyroxine) is the main hormone secreted by the thyroid but is less potent. T3 (triiodothyronine) is 3-4 times more potent and is mostly produced by peripheral conversion of T4 in body tissues.