with regard to thyroid physiology

Correct Answer: C

Rationale: T4 (thyroxine) is synthesized from tyrosine in thyroglobulin within thyroid follicles, iodinated to form T3/T4, released to boost metabolism. Metabolism occurs mainly in liver/kidneys, not spleen/bone marrow. T3/T4 act via nuclear receptors, not identical membrane ones T3 is more potent. T4 isn't more active converted to T3 for greater effect. Thyroglobulin's role as a precursor scaffold distinguishes T4 synthesis, key to thyroid hormone production, unlike incorrect metabolism sites, receptor action, or potency claims.

Correct Answer: B

Rationale: Free cortisol feeds back negatively on both pituitary (ACTH) and hypothalamus (CRH) false to exclude hypothalamus. Suprachiasmatic nuclei drive ACTH's diurnal rhythm. Haemorrhage boosts aldosterone (RAAS) and cortisol (stress). Testosterone inhibits LH. Hypothalamic feedback distinguishes cortisol's regulation, key to HPA axis, unlike rhythm, stress, or gonadal truths.

Correct Answer: A

Rationale: PTH decreases serum PO₄ by increasing renal excretion, raising Ca²⁺ net effect is PO₄ reduction. Vitamin D increases Ca/PO₄ absorption, reducing excretion opposite claim. Calcitonin is from thyroid C-cells, not parathyroid. Insulin promotes bone formation (anabolic), not decreases. PTH's PO₄-lowering distinguishes it, key to calcium-phosphate regulation, unlike vitamin D, calcitonin, or insulin errors.

Correct Answer: C

Rationale: Oestrogen inhibits osteoclasts, reducing bone resorption, preserving mass post-menopause osteoporosis link. Ca is ~1-2% body mass, not 15%. Intestinal Ca absorption is active (vitamin D), not passive. Corticosteroids inhibit osteoblasts, promoting loss. Oestrogen's osteoclast suppression distinguishes it, key to bone health, unlike mass, absorption, or steroid errors.

Correct Answer: D

Rationale: Local hormones and paracrine hormones act on nearby cells without bloodstream travel e.g., prostaglandins (local) or growth factors (paracrine). Autocrine hormones target the secreting cell, circulating hormones travel via blood (e.g., insulin). 'Both 1 and 2' (local and paracrine) fits, as both share this localized action. This distinguishes short-range endocrine signaling, key to tissue-specific responses like inflammation, contrasting with systemic hormone effects.