Chapter 28.
Answers A, C, and D are true statements, whereas B and E are false.
1. Hashimotos thyroiditis is a probable diagnosis, because the Microsomal auto-antibodies have caused atrophy of the thyroid and hypothyroidism (myxoedema). A high TSH level and a low total or free T4 in plasma confirms the diagnosis primary hypothyroidism. Hypercholesterolaemia is typical as is increased concentrations of liver and muscle enzymes (aspartate transferase, creatine kinase) in the plasma.
2. Thyroid hormone is the form of levo-thyroxine is the treatment of choice. The initial dose to old patients has to be cautiously low such as 50 mg orally once or twice a week, since any overdose elicits cardiac arrhythmia or failure.
3. A patient with one organ-specific autoimmune disorder (here pernicious anaemia) is prone to get another (Hashimotos thyroiditis with atrophy and myxoedema).
1. The specific activity (SA = c*/C) of radio-iodide (131I) in the urine is:
1.6 × 107/(0.2 × 65) = 12 308 Bq/mg.
Since SA is the same in urine and in plasma, it follows that the concentration of iodide in plasma is: Activity per ml/SA.
Start-concentration = 38 300/12 308 (Bq/l)/(Bq/mg) = 3.11 mg per l.
Compared to the physical half-life of 8 days (192 hours) the test period is short. Assuming first order disintegration of iodine the SA must be adjusted for disintegration during the experiment, which is two hours of the normal half-life (192 hours). k = 0.693/192 = 0.0036 h-1. SA2 = SA0 e-2•0.0036.
SA2 = 12 308* e-0.0072 = 12 220 Bq per mg.
At the end of the 2 hour's test, the plasma [iodide] is (26 100/12 220) = 2.1 mg per l.
2. Secretion of organic bound iodide from the thyroid gland is negligible during the test. The net uptake of iodide in the thyroid gland is equal to the rise in activity divided by SA: 77 500/12 220 Bq/(Bq/mg).
The result is 6.34 mg or 3.17 mg per hour, which is above the normal average of 2.4 mg per hour and suggestive of the diagnosis hyperthyroidism.
3. The thyroid plasma clearance for iodide equals its absorption flux divided by its plasma concentration: (6.34/120)/(3.11 × 10-3) = 17 ml plasma per min. - This result is higher than the predicted value of 10 ml per min.
4. Plasma iodide is cleared only through the kidneys and through the thyroid gland. The total iodide clearance must be the renal plus the thyroid plasma clearance: (36 + 17) = 53 ml/min. The amount of iodide eliminated per min is a constant fraction (k) of the amount remaining in the body. This is exponential elimination with a rate constant k. The distribution volume for iodide is 12 l.
k = 53/12 000 = 0.00441667 min-1.
5. The biological half-life (T½) is equal to (ln 2)/k or 0.693/0.0037857 = 153 min. The physical half-life for 131I is 192 hours or 8 days.