Step by Step
1
T3 and T4 production
The thyroid gland's follicular cells produce T3 (triiodothyronine, with 3 iodine atoms) and T4 (thyroxine, with 4 iodine atoms) from thyroglobulin combined with iodine. T4 acts as a prohormone, converted to the more active T3 in peripheral tissues by an enzyme called deiodinase.
2
The effects of thyroid hormone
Thyroid hormones increase basal metabolic rate, increase heart rate and cardiac output, are essential for normal brain development (deficiency during infancy causes cretinism), promote bone growth and maturation, and have a permissive effect for growth hormone's actions.
3
Control and the other thyroid hormone: calcitonin
TSH from the pituitary controls thyroid hormone production. Separately, parafollicular (C) cells in the thyroid produce calcitonin: when blood calcium rises, calcitonin is released, inhibiting osteoclasts and lowering blood calcium — though this plays only a minor role in adults, being more significant in children.
4
Clinical disorders
Hypothyroidism (decreased T3/T4) causes myxedema, weight gain, and cold intolerance. Hyperthyroidism (increased T3/T4) is classically caused by Graves' disease, where antibodies against the TSH receptor overstimulate the thyroid.
Applied Walkthrough
1
An infant born with untreated congenital hypothyroidism (insufficient T3/T4) fails to develop normal brain function — illustrating why thyroid hormone is considered essential for normal development, not just metabolic regulation.
2
In a healthy adult, TSH from the pituitary regulates ongoing T3/T4 production, which in turn raises basal metabolic rate and heart rate — maintaining normal energy metabolism.
3
A patient with Graves' disease develops antibodies that mimic TSH, chronically overstimulating the thyroid and producing excess T3/T4 — leading to hyperthyroidism, with symptoms like weight loss, heat intolerance, and rapid heart rate.
4
Separately, if this same patient's blood calcium rises for an unrelated reason, their thyroid's parafollicular C cells would release calcitonin, providing a modest calcium-lowering effect distinct from the T3/T4 pathway entirely.
Exam Application
Exams test whether you understand that T4 is a prohormone converted to more active T3, whether you know the wide-ranging effects of thyroid hormone (metabolism, development, bone growth), and whether you can distinguish hypothyroidism from hyperthyroidism, including their classic clinical causes.
⚠ Common Trap
The most common trap is assuming T4 and T3 are equally potent — T4 is actually a prohormone that must be converted (via deiodinase) into the more biologically active T3 in peripheral tissues.
✓ Quick Self-Check
1. What are T3 and T4 made from?
Thyroglobulin combined with iodine.
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2. Which is more biologically active — T3 or T4?
T3 — T4 is a prohormone converted to T3 in peripheral tissues.
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3. What hormone controls thyroid hormone production, and where is it from?
TSH, from the anterior pituitary.
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4. What does calcitonin do, and which cells produce it?
Lowers blood calcium by inhibiting osteoclasts; produced by parafollicular (C) cells.
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5. What is the classic cause of hyperthyroidism, and how does it work?
Graves' disease, caused by antibodies against the TSH receptor that overstimulate the thyroid.
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