Lost a planet...how embarrassing.

Week 6: Endocrine/Reproductive

Thyroid

Updated by Tracey 16 August 02

The thyroid gland, anterior to the trachea below the larynx, is stimulated by thyroid stimulating hormone from the anterior pituitary, which is in turn controlled by thyrotropin-releasing hormone (TRH) from the hypothalamus. Secretes thyroxine and triiodothyronine which effect the body's metabolic rate, and calcitonin which is important for calcium metabolism.

Thyroxine and Triiodothyronine

dentical function, but different rapidity and intensity of action. Triiodothyronine secretion is much less, and persists for a much shorter time, but is about 4 times as potent as thyroxine.

Formation and Storage: in Golgi complex and ER ...
Iodide ion and tyrosine combine to form monoiodotyrosine and then diiodotyrosine. Two diiodotyrones form thyroxine, one mono and one di form triiodothyronine. Stored in a thyroglobulin molecule. Stored amount can supply body for normal requirements for 2-3 months.

In the blood:
bind with plasma proteins. Half the thyroxine is released to the tissue cells every 6 days; halflife of triiodothyronine in the blood is 1 day. In the tissues, they are stored and used slowly over days or weeks. Increased activity from thyroxine injections takes 2-3 days -- latency period -- increase is progressive, maxing at 10-12 days, then decreasing. Triiodothyronine acts about 4 times as quickly, with latent period of 6-12 hours and maximum activity in 2-3 days.

Functions in the tissues:
general effect of increasing transcription rates, increasing protein enzyme, structural and transport proteins and other substances, resulting in increased functional activity through the body. Thyroxine is converted into triiodothyronine and binds to thyroid hormone receptors that are attached or in close proximity to DNA strands, which initiate transcription.

Controls metabolic activity, seen in metabolic rate: fat and carbohydrate metabolism, O2 utilization and dilation, heart rate, respiration, GI secretions and motility. Plasma cholesterol, phopholipid and triglyceride levels. Preprubescent growth rates, and fetal and postnatal brain dev.

Abnormalities:

Hyperthyroidism: gland size 2-3x normal, with secretion rates also increased. Globulin antibodies acting similarly to TSH bind with TSH receptors continually activating the cells. Symptoms include those of excessive amounts, including excitability, heat intolerance, increased sweating, weight loss, diarrhea, muscular fatigue/weakness, nervousness, fatigue with inability to sleep, hand tremors. Treatments: removal or thyroid-blocking drug propylthiouracil.
Hypothyroidism: effects generally opposite those of hyperthyroidism.
Endemic colloid goiter (goiter means enlarged thyroid gland) can result from insufficient iodine which halts thyroid hormone production. Pituitary keeps secreting TSH, resulting in LARGE amounts of thyroglobulin to accumulate. Idiopathic colloid goiter develops when there is sufficient iodine, but there may be mild inflammation in the thyroid gland itself so TSH accumulates in non inflamed portions. Hypothyroidism can also result from irradiation, removal or destruction of the gland. Symptoms include somnolence, extreme muscular sluggishness, decreased heart rate and cardiac output and blood volume, increased weight, constipation, mental sluggishness, depressed hair growth or scaly skin, and myxedema (edematous appearance through the body). Also arteriosclerosis from increased blood lipids. Treatment of thyroxine or dessicated thyroid gland.