1. Majority of the circulating $T_3$ remains in bound form.
2. Only 50% of the circulating $T_3$ is secreted by thyroid.
3. TSH estimation is not a sensitive test for diagnosis of primary hypothyroidism.
4. Fetal pituitary-thyroid axis is dependent to a large extent on maternal pituitary-thyroid axis.
Which of the statements given above is/are correct ?
Thyroid $T_3$ Binding: Majority Bound
Thyroid hormones, including Triiodothyronine ($T_3$), circulate in the blood largely attached to specific binding proteins (like TBG) and albumin. Only a minimal fraction, approximately 0.3%, remains free ($T_3$) and biologically active. Thus, the assertion that the majority of circulating $T_3$ is bound is accurate.
$T_3$ Secretion Origin: Thyroid vs Peripheral
The thyroid gland directly secretes about 20-30% of the total circulating $T_3$. The larger portion, roughly 70-80%, originates from the peripheral deiodination (conversion) of Thyroxine ($T_4$) in tissues such as the liver and kidneys. The statement "Only 50% of the circulating $T_3$ is secreted by thyroid" is considered incorrect in the context of this question.
TSH Test Sensitivity in Primary Hypothyroidism
In primary hypothyroidism, the thyroid gland's function is impaired, leading to decreased production of thyroid hormones ($T_3$ and $T_4$). This triggers the pituitary gland to increase the secretion of Thyroid Stimulating Hormone (TSH) in an attempt to stimulate the thyroid. Consequently, an elevated TSH level is the earliest and most sensitive indicator of primary hypothyroidism. Therefore, the statement that TSH estimation is not sensitive is false.
Fetal Pituitary-Thyroid Axis Dependence
The fetal hypothalamic-pituitary-thyroid (HPT) axis develops and matures largely autonomously throughout gestation. While maternal thyroid hormones ($T_4$) readily cross the placenta and are essential for normal fetal development, particularly brain development, the functional independence of the fetal axis means it is not significantly dependent on the maternal axis's activity. Thus, the statement suggesting substantial dependence is incorrect.
Conclusion: Correct Thyroid Hormone Statements
Summarizing the analysis:
- Statement 1 is correct.
- Statement 2 is incorrect.
- Statement 3 is incorrect.
- Statement 4 is incorrect.
Therefore, only statement 1 is correct.
Important Questions from Thyroid Disorders
- Which one of the following diseases is characterized by destructive lymphoid infiltration of the thyroid gland leading to fibrosis and enlargement of the thyroid gland with an increased risk of developing thyroid lymphoma?
With regards to treatment of thyrotoxicosis of Grave's disease, consider the following statements:
1. Propylthiouracil is suitable in breastfeeding patients.
2. Large goitre is an indication for thyroidectomy.
3. Radioiodine is recommended for management in pregnancy.
4. Agranulocytosis is a complication associated with carbimazole.
Which of the statements given above are correct?
With regard to management of hypothyroidism, consider the following statements:
1. Treatment is with levothyroxine replacement.
2. It is taken as a single daily dose.
3. Average dose is 1.6 $\mu$gm/kg/day.
4. Full dose per weight should be started in those with coronary artery disease.
Which of the statements given above are correct?
- Low serum thyroglobulin levels are seen in
- A 27-year lady underwent thyroidectomy for thyroid carcinoma. Post-operatively she started developing muscle cramps, perioral and peripheral paresthesia, and carpopedal spasm. Which one of the following interventions will be appropriate for this patient?
