Question
Compare hypothyroidism and hyperthyroidism in terms of causes and symptoms. What is goitre, and how is it caused by iodine deficiency? What is cretinism?
(NCERT Class 11 — frequently asked in NEET)
Solution — Step by Step
The thyroid gland produces two hormones:
- T3 (triiodothyronine) — more active form
- T4 (thyroxine) — produced in larger quantities, converted to T3 in tissues
Both require iodine for synthesis. They regulate basal metabolic rate (BMR), growth, development, and body temperature.
Regulated by the hypothalamus-pituitary axis: Hypothalamus → TRH → Anterior pituitary → TSH → Thyroid → T3/T4 (negative feedback inhibits TSH).
Causes: Iodine deficiency, autoimmune destruction (Hashimoto’s thyroiditis), surgical removal
Symptoms:
- Low BMR → weight gain, lethargy, fatigue
- Cold intolerance
- Slow heart rate
- Dry skin, hair loss
- Myxoedema (in adults) — puffy face and hands due to accumulation of mucopolysaccharides under the skin
Cretinism: Hypothyroidism in infants/children → stunted growth, mental retardation, delayed sexual maturation. Irreversible if not treated early.
Causes: Graves’ disease (autoimmune — antibodies mimic TSH and stimulate thyroid), thyroid tumour
Symptoms:
- High BMR → weight loss despite good appetite
- Heat intolerance, excessive sweating
- Rapid heart rate (tachycardia), anxiety, tremors
- Exophthalmos (bulging eyes) — characteristic of Graves’ disease
Goitre is the visible swelling of the thyroid gland in the neck.
Simple goitre (iodine deficiency): Without iodine, the thyroid can’t make T3/T4. Low T3/T4 means no negative feedback on the pituitary → TSH keeps rising → continuously stimulates the thyroid → gland enlarges trying to produce more hormone. This is endemic goitre, common in hilly regions far from the sea (where iodine in soil/water is low).
Toxic goitre: In Graves’ disease, the gland enlarges due to overstimulation by autoimmune antibodies.
Prevention: Iodised salt — adding potassium iodide to table salt prevents iodine deficiency goitre.
Why This Works
The thyroid system is a classic negative feedback loop. When T3/T4 levels drop, the pituitary releases more TSH to push the thyroid harder. If the thyroid can’t respond (due to lack of iodine), TSH keeps climbing, and the gland hypertrophies — that’s goitre.
In hyperthyroidism (Graves’), the feedback loop is broken — autoimmune antibodies stimulate the thyroid independent of TSH levels. The gland doesn’t know when to stop, producing excess hormones.
Alternative Method — Quick Comparison Table
| Feature | Hypothyroidism | Hyperthyroidism |
|---|---|---|
| Hormone level | Low T3/T4 | High T3/T4 |
| BMR | Decreased | Increased |
| Weight | Gain | Loss |
| Heart rate | Slow | Fast |
| Temperature tolerance | Cold intolerant | Heat intolerant |
| Specific condition | Myxoedema (adult), Cretinism (child) | Graves’ disease (exophthalmos) |
For NEET: remember the trio — cretinism (hypothyroidism in infants), myxoedema (hypothyroidism in adults), and Graves’ disease (hyperthyroidism with exophthalmos). These are the three conditions NEET tests most from this topic.
Common Mistake
Students say “goitre means hyperthyroidism.” Wrong — goitre can occur in BOTH hypothyroidism (iodine deficiency goitre) and hyperthyroidism (Graves’ disease). Goitre simply means an enlarged thyroid — it doesn’t tell you whether the hormone level is high or low. Always check the cause before linking goitre to hypo or hyper.