Physiological Actions And Clinical Investigations In Determination Of Thyroid Hormones
Calcitonin Effect On The Human Bone
A General Overview
Actions Of thyroid Hormones
- Increase in cellular oxidation in all tissues
- Increase in protein breakdown
- Increase the turnover of carbohydrates and lipids
- Calcium is mobilized from bone
- The cardiovascular effects consist of increase in the heart rate and sensitization of the beta-adrenergic receptors to the action of catecholamines.
Calcitonin: Calcitonin is a peptide hormone formed in the parafollicular cells (C cells) of the thyroid. Calcitonin inhibits bone resorption and the release of calcium from bone, thereby helpting to lower calcium levels in blood. Catabolism of bone is significantly reduced. Levels of calcitonin increase during hypercalcemia and the secretion stops when the calcium levels falls. High levels of calcitonin are seen in patients with medullary carcinoma of the thyroid. Still in this condition, there is no hypocalcemia. Calcitonin has found therapeutic use in the management of acute hypercalcemia, osteoporosis and Paget’s disease of bone.
Assessment of thyroid function: Clinical examination is most valuable in all cases. Though, several classic features such as lid lag and thyroid bruit specifically point to florid hyperthyroidism, in marginal cases, the clinical symptomatology shows considerable overlap with normals or other non-thyroid disorders. Many tests are available to assess thyroid function but no single test is totally satisfactory. Thyroid function tests may be broadly classified into radio-isotopic methods and non-radioisotopic methods.
1. Basal metabolic rate
1. I131 uptake, TSH stimulation and T3 suppression
2. Estimation of serum protein bound iodine (PBI) and butanol extractable iodine (BEI)
2. I131 PBI estimation
3. Estimation of cholesterol, creatinine and creatine phosphokinase (CPK)
3. T3 resin uptake
4. Demonstration of thyroid autoantibodies
4. RIA of T3, T4 and TSH
5. Thyroid biopsy
5. TRH stimulation 6. Thyroid scan.
Laboratory Investigations Of Thyroid Hormones
More Clinical Investigations
Protein-bound Iodine (PBI): Estimation of PBI was used to indicate thyroid function before more specific tests became available. The presence of exogenous substances such as organic iodides and radiographic contrast media increases PBI values. Hence this estimation is seldom done at present.
PBI131 estimation: After the oral administration of I131, the percentage of the administered dose bound to protein is estimated. Normal value is 0.4%.
Butanole extractable Iodine (BEI) estimation: This method eliminates the influence of non-hormonal iodides in serum PBI estimation. Since iodinated proteins like monoiodotyrosines and diiodotyrosines and inorganic iodides are insoluble in butanol. Only T3 and T4 are extracted by this solvent. Unfortunately most of the exogenous organic iodides such as radiographic contrast dyes are soluble in butanole and hence this test is also only of limited value.
Plasma transport: Around 60% of thyroxine (T4) circulates in blood bound to thyroid binding globulin (TBG) which is an alpha-globulin, 30% is bound to pre-albumin and the rest to albumin. Tri-iodothyronine (T3) is bound only to TBG and albumin and the binding is less firm when compared to that of T4. 0.1% of T4 and 1% T3 remain unbound (free form) in the plasma. The total plasma level of T4 is 4-8 ug/dl and that of T3 is 150-250 ng/dl. The sum total of T4 and T3 bound to the carrier protein is referred to as plasma protein-bound iodine (PBI). The normal range is 4-8 ug/dl. The levels of T4 and T3 are regulated by the TSH from the anterior pituitary and TRH of the hypothalamus. Rising levels of thyroid hormones inhibit TSH secretion. Both T4 and T3 are de-iodinated in the hepatic cells by the microsomal enzymes and the iodides are excreted in urine (70ug/day).
© 2014 Funom Theophilus Makama