In primary hyposecretion of thyroid hormones
a. levels of thyroid hormones in the blood decrease
b. levels of trh in the blood decrease.
c. levels of tsh in the blood decrease.
d. all of the above are true.
e. none of the above is true

A. Levels of thyroid hormones in the blood decreases.

Explanation:

Hyposecretion means too little hormone levels. It can be primary hyposecretion which is due to damage to the gland or it can be secondary hyposecretion which is due to deficiency of tropic hormones.

Tropic hormones: If a hormone produced from one endocrine gland acts on another endocrine gland as their target then we call such hormones as Tropic hormones.

For example, Hypothalamus produces the TRH i.e. thyrotropin releasing hormone which stimulates the anterior pituitary to produce TSH i.e. thyroid stimulating hormone. Here both TRH and TSH are examples of tropic hormones. TRH is produced by hypothalamus and stimulate pituitary gland, similarly TSH is produced by pituitary gland and acts on thyroid gland to stimulate the release of T3 and T4.  

In case of primary hyposecretion, there can be damage of the thyroid gland which may result in decreased levels of T3 (triodotyronine) and T4 (tetraiodothyronine) in the blood, but there is no change in the tropic hormone levels i.e. TSH or TRH levels.

Hypothyroidism is mainly due to deficiency of iodine in the diet or damage to the thyroid gland.  

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