THYROID HORMONE – METABOLISM, TRANSPORT & REGULATION.
| A | Glycine | |
| B |
Tyrosine |
|
| C |
Phenylalanine |
|
| D |
Iodine |
Thyroid hormone formation involves many steps. Which of the following amino acid is the precursor for thyroxine?
| A |
Glycine |
|
| B |
Tyrosine |
|
| C |
Phenylalanine |
|
| D |
Iodine |
Tyrosine:
- Tyrosine is the precursor of triiodothyronine and thyroxine.
- Neural cells convert tyrosine to epinephrine and norepinephrine.
- While dopa is also an intermediate in the formation of melanin, different enzymes hydroxylate tyrosine in melanocytes.
- Dopa decarboxylase, a pyridoxal phosphate-dependent enzyme, forms dopamine.
- Subsequent hydroxylation by dopamine -oxidase then forms norepinephrine.
- In the adrenal medulla, phenylethanolamine-N-methyltransferase utilizes S-adenosylmethionine to methylate the primary amine of norepinephrine, forming epinephrine.
Ref: Rodwell V.W. (2011). Chapter 30. Conversion of Amino Acids to Specialized Products. In D.A. Bender, K.M. Botham, P.A. Weil, P.J. Kennelly, R.K. Murray, V.W. Rodwell (Eds), Harper’s Illustrated Biochemistry, 29e.
Among the following all are hydrophilic hormones that act on cytosolic receptors except one which is a lipophilic hormone that acts on nuclear receptor:
| A |
Thyroxine |
|
| B |
Epinephrine |
|
| C |
GH |
|
| D |
ACTH |
A i.e. Thyroxine
All of the following hormones have cell surface receptors except –
| A |
Adrenaline |
|
| B |
Growth Hormone |
|
| C |
Insulin |
|
| D |
Thyroxine |
Ans. is ‘d’ i.e., Thyroxine
o Thyroxine acts on intracellular receptors.
| A |
Albumin |
|
| B |
Prealbumin |
|
| C |
Globulin |
|
| D |
Transferrin |
Ans. is ‘c’ i.e., Globulin
Most of the circulating thyroid, hormones are protein bound, leaving only 0.02% T4 and about 0.2% T3 unbound or free.
There are three hormone binding plasma proteins, i.e., thyroxine binding globulin (major thyroid hormone binding protein), thyroxine binding albumin and thyroxine binding prealbumin.
Thyroid binding globulin level is raised in pregnancy and by drugs estrogens, methadone, heroin, tranquilizers, clofibrate.
TBG decreases by glucocorticoid, androgen, danazol, asparginase (cancer chemotherapy) and nephrotic syndrome.
| A |
Insulin |
|
| B |
T3 |
|
| C |
T4 |
|
| D |
b and c both |
Ans. is `b & c’ i.e., T3 & T4
Which of the following is an amide hormone?
| A |
Thyroid releasing hormone |
|
| B |
Melanocyte stimulating hormone |
|
| C |
ACTH |
|
| D |
TSH |
Ans. is ‘a’ i.e., Thyroid releasing hormone
Thyroid Releasing Hormone
It is the hormone secreted by the nerve endings in the median eminence of hypothalamus.
It is the n transported to anterior pitutary by hypothalamohypophyseal portal system.
Thyroid releasing hormone in the pure form is a tripeptide amide – Pyroglutamylhisdidynylproline amide.
