Metabolism of magnesium and selected minerals in hypothyroid sheep

The effect of ¹³¹I induced hypothyroidism on magnesium, calcium, potassium and phosphorus metabolism was investigated in three balance trials with ten sheep. Both ²⁸Mg and ⁴⁷Ca cleared the plasma at a slower rate in hypothyroid than in control sheep after intravenous administration. Early appearance of ²⁸Mg in blood after oral administration suggested significant absorption from the rumen in both groups. Rate of absorption was not influenced by treatment. Plasma stable magnesium was 20 percent (P<.01) and phosphorus 23 percent (P<.0l) above control levels but calcium and potassium levels in plasma were not changed by thyroid alteration. Hypothyroidism reduced feed consumption, fecal weights and urinary volumes. Absorption of dietary magnesium was elevated in the thyroid-altered sheep, and calculated fecal endogenous losses were reduced for both magnesium and calcium. Percentage excretions of dietary magnesium, calcium and potassium were reduced in feces but increased in urine. Available body pools of both magnesium and calcium were reduced in hypothyroid sheep. Although total skeletal magnesium content was not affected by thyroid alteration, calcium (P<.05) and phosphorus (P<.10) were lower when expressed on a body weight basis.